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What a MIME type controls in WordPress
A MIME type describes the format of a file, while the extension is the suffix in its name. A JSON file normally has the .json extension and the application/json MIME value. WordPress uses an extension-to-MIME map when it decides whether an upload is acceptable; the extension alone is not the complete security decision.
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The core map contains common image, audio, video, document, archive, and OpenDocument formats. Its contents can change with WordPress releases, so code should inspect the current map rather than assume that a type is permanently present.
wp_get_mime_types()returns the general extension-to-MIME map.get_allowed_mime_types()returns the types currently allowed for an upload context.wp_check_filetype( $filename, $mimes = null )matches a filename against a map and returnsextandtype. If there is no match, those values are false.wp_check_filetype_and_ext( $file, $filename, $mimes = null )performs the mapping and, for images, tries to determine the real MIME type. It can return a corrected image filename when the extension does not match the content.
That distinction matters: adding a name to a list does not make arbitrary bytes safe. A secure implementation allows only the formats the site needs and validates the uploaded content before it is stored or processed.
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Choose the right WordPress hook
| Hook or setting | What it changes | Best use | Important limitation |
|---|---|---|---|
upload_mimes |
The upload allowlist used by a site | Adding one or more extensions, or removing an extension from uploads | It does not by itself prove that file bytes match the claimed format |
mime_types |
The general extension-to-MIME map | Adding a mapping that other WordPress code can read | WordPress documents this filter for adding types, not for removing them; use upload_mimes to restrict uploads |
| Multisite network setting | A network-wide ceiling on uploadable extensions | Letting a Super Admin control what individual sites may upload | A site-level filter cannot widen a type that the network has excluded |
| Form, security, server, or host rule | An additional layer outside core upload handling | Diagnosing failures after WordPress accepts the type | Each layer has its own configuration and logging |
For a normal site-level change, start with upload_mimes. Use mime_types only when you intentionally need to extend the general map for code that reads it independently of the upload allowlist.
Add one MIME type with upload_mimes
Put a narrow rule in a small custom plugin or another site-specific functionality location. That keeps the change when the active theme changes and makes it possible to audit or remove the rule without editing WordPress core.
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/**
* Plugin Name: Site upload MIME rules
*/
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['json'] = 'application/json';
return $mimes;
} );Save the file as a PHP plugin in the site’s wp-content/plugins directory, activate it in the Plugins screen, and replace the example pair with the exact extension and MIME value required by your format. Do not add a wildcard or a rule that accepts every extension.
If the functionality is already managed in a site-specific plugin, add the filter there instead of creating a second plugin. A child theme’s functions.php can also hold the filter, but a theme change then removes the behavior; a functionality plugin is usually easier to audit and deploy.
Allow several documented formats in one rule
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['json'] = 'application/json';
$mimes['csv'] = 'text/csv';
$mimes['svg'] = 'image/svg+xml';
$mimes['webp'] = 'image/webp';
return $mimes;
} );Keep each entry explicit. If a format has more than one legitimate extension, add each extension deliberately and document why it is needed. Do not assume that a browser’s reported content type is trustworthy; WordPress still needs to inspect the file during the upload process.
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Inspect the active map before changing code
A frequent cause of confusion is testing against a map that is different from the one used by the upload request. You can inspect the effective allowlist from a temporary administrative diagnostic, a controlled debugging endpoint, or a command-line bootstrap that loads WordPress:
$allowed = get_allowed_mime_types();
error_log( print_r( $allowed, true ) );Search the resulting array for the extension you need. The keys may contain several extensions separated by a pipe character, so look for the exact entry rather than assuming one key equals one suffix. Remove diagnostic output after testing; exposing the complete list to unauthenticated visitors is unnecessary.
To check a filename against a supplied map:
$result = wp_check_filetype( 'sample.json', get_allowed_mime_types() );
// $result['ext'] is 'json' and $result['type'] is 'application/json'
// when the current map permits that pair.A false ext or type means that the filename did not match the map passed to the function. It does not prove that the file is corrupt; it indicates that the current allowlist or extension is wrong.
Fix “Sorry, this file type is not permitted for security reasons”
Work through the checks in order. Changing several layers at once makes it difficult to identify the real restriction.
1. Confirm the pair you intend to allow
- Write down the exact extension, without the leading dot, and the MIME string required by the format.
- Check that the filename has only the expected final extension. A name such as
report.json.exemust not be treated as JSON. - Confirm that your filter is loaded on the request that performs the upload. A plugin that is inactive, placed in the wrong directory, or guarded by an admin-only condition will not change the allowlist.
2. Check the effective allowlist
Use get_allowed_mime_types() in a controlled diagnostic and verify that the extension appears with the intended MIME value. If it is absent, correct the filter before investigating the file itself.
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3. Check Multisite network restrictions
On Multisite, WordPress applies the network’s permitted extension list as an additional limit. A Super Admin should open Network Admin → Settings → Upload file types and add the extension there if network policy allows it. Core’s check_upload_mimes() intersects the general map with the network upload_filetypes option. The documented default for that option is jpg jpeg png gif; a site-level filter cannot expand a network list that omits the requested type.
4. Check the bytes and image metadata
If the extension appears in the map but the upload is rejected, compare the file’s real contents with its name. WordPress’s wp_check_filetype_and_ext() performs the normal mapping and, for images, attempts to determine the actual MIME type. It can reject an extension/content mismatch or return a corrected filename for an image.
if ( ! function_exists( 'wp_check_filetype_and_ext' ) ) {
require_once ABSPATH . 'wp-admin/includes/file.php';
}
$check = wp_check_filetype_and_ext(
$path_to_uploaded_file,
$original_filename,
get_allowed_mime_types()
);
if ( empty( $check['ext'] ) || empty( $check['type'] ) ) {
// Reject the upload and record a useful, private diagnostic.
}The include is important when this function is called from a plugin or theme and the function has not already been loaded.
5. Look beyond WordPress core
A form builder may maintain its own extension list. A security plugin, web server, reverse proxy, or hosting policy may also block a type after WordPress has accepted it. Compare the browser’s error with the PHP error log and the server response, and review those products’ allowlists one at a time. Do not weaken WordPress validation merely to bypass an unrelated layer.
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Validate custom uploads securely
The media-library path is not the only place files enter a site. A custom form or REST endpoint must perform its own checks before moving a temporary file into permanent storage.
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- Sanitize the name first. Apply WordPress filename sanitization before examining the extension. This removes unsafe characters and reduces ambiguity in names supplied by a user.
- Use an allowlist. Accept only the exact extensions and MIME values required by the feature. An “allow everything” policy turns a convenience fix into an execution and content-delivery risk.
- Check both name and content. Use
wp_check_filetype()for the extension-to-MIME decision and preferwp_check_filetype_and_ext()when image content needs verification. - Keep storage and permissions appropriate. A file that contains markup or script-like content needs additional sanitization and access controls; merely labeling it with an image MIME type is not a sanitizer.
- Log rejections without exposing internals. Record the extension, detected type, and reason in a private log, but return a generic message to an unauthenticated uploader.
Do not enable ALLOW_UNFILTERED_UPLOADS for a routine fix. That constant bypasses the narrow allowlist model and should not be a substitute for identifying the one format the feature actually needs.
Common recipes and their cautions
JSON
For configuration or data files, the usual pair is json and application/json. Keep access controls in mind: JSON can disclose application data even though it is not normally executable.
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['json'] = 'application/json';
return $mimes;
} );CSV
Comma-separated files commonly use csv and text/csv. If the file will later be opened in a spreadsheet, treat imported cell content as untrusted data and validate it in the importing feature; allowing the upload does not validate rows or formulas.
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add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['csv'] = 'text/csv';
return $mimes;
} );WebP
WebP images use webp and image/webp. Confirm that the installed WordPress and image-processing stack can generate the sizes your site needs after the file is accepted. If the file is renamed from another format, content validation still has to pass.
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['webp'] = 'image/webp';
return $mimes;
} );SVG and SVGZ
SVG is XML and can contain active or externally referenced content; SVGZ is its compressed form. If you decide to permit either, add only the required extension and put sanitization, who-can-upload rules, and serving controls in place. A MIME filter alone does not make an SVG safe.
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add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['svg'] = 'image/svg+xml';
$mimes['svgz'] = 'image/svg+xml';
return $mimes;
} );When to use mime_types instead
WordPress describes mime_types as a filter for adding entries to the general MIME map. Its documented guidance is: “This filter should be used to add, not remove, mime types. To remove mime types, use the ‘upload_mimes’ filter.” That makes it useful when another part of your code calls wp_get_mime_types() and needs to see a new mapping, but it is not the usual control for restricting uploads.
add_filter( 'mime_types', function ( $mimes ) {
$mimes['json'] = 'application/json';
return $mimes;
} );Adding the same pair to both filters is usually unnecessary. Start with the upload filter, then add the general-map filter only when a specific integration demonstrably requires it.
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| Approach | Scope | Map or allowlist | Content validation | Who can upload | Removal and audit |
|---|---|---|---|---|---|
Site-specific plugin using upload_mimes |
One site | Changes the upload allowlist | Use WordPress validation functions separately | Any role already permitted to upload, subject to WordPress capabilities | Deactivate or delete one documented rule |
| Theme or child-theme code | One site while that theme is active | Changes the upload allowlist | Use WordPress validation functions separately | Existing upload-capable users | Can disappear during a theme change; harder to audit as site functionality |
| Network setting on Multisite | All sites in the network | Caps the extensions available to sites | Core still performs its normal checks | Controlled by each site’s capabilities after the network permits the type | Changed by a Super Admin in Network Admin |
| Generic MIME-management plugin | Depends on its configuration; may be site or network wide | May alter the map, the upload allowlist, or both | Verify whether it checks file contents; do not assume it does | Depends on plugin and WordPress capabilities | Review settings, exported configuration, and plugin deactivation behavior |
| Hosting or web-server rule | Server, virtual host, or account | Usually an additional restriction | Often outside WordPress’s validation functions | All requests reaching that layer | Requires server-level documentation and change control |
Test the change without weakening security
- Back up the plugin or site-specific code and note the original allowlist.
- Activate the narrow filter and reload the upload screen.
- Upload a genuine sample of the intended format, not a file that was merely renamed.
- Confirm that WordPress stores the file, records the expected extension and MIME value, and generates any required metadata.
- Try a deliberately mismatched sample in a controlled environment and confirm that validation rejects it.
- Repeat the test as the roles that should and should not have access, especially on Multisite.
- Review PHP and web-server logs, then remove temporary diagnostics and test files.
For a production rollout, record the extension, MIME value, business reason, approving administrator, date, and removal plan. That small audit trail prevents an old exception from becoming a permanent, forgotten attack surface.
Remove or roll back a MIME change
Delete the specific array entry from the plugin, or deactivate the plugin that owns it, then clear any object or page cache that could leave an old upload form visible. If the type was also added to a Multisite network’s upload list, a Super Admin must remove it there separately. Existing files are not automatically deleted when a MIME rule is removed; decide how those files should be retained, quarantined, or served before changing the policy.
The safest long-term configuration is therefore small and explicit: one documented filter, one required extension/MIME pair, normal WordPress content checks, and a network-level review when Multisite is involved.
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