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Use WordPress’s upload_mimes filter to allow one additional extension and map it to the file’s real MIME type. Put the code in a small custom plugin or mu-plugin so it survives theme updates. Do not enable unrestricted uploads: ALLOW_UNFILTERED_UPLOADS can permit PHP and other executable files.
Add one file type with a custom plugin
The safest general approach is a narrowly scoped MIME mapping. This example allows SVG files:
<?php
/**
* Plugin Name: Site Upload MIME Types
*/
function my_custom_mime_types( $mimes ) {
$mimes['svg'] = 'image/svg+xml';
return $mimes;
}
add_filter( 'upload_mimes', 'my_custom_mime_types' );Create a PHP file containing the code, place it in wp-content/plugins/, and activate it under Plugins → Installed Plugins. A site-specific or custom plugin is update-safe; editing a parent theme is not, because a theme update can overwrite the change. An mu-plugin in wp-content/mu-plugins/ is also suitable when the rule should always be active.
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Change the extension and MIME value
Replace svg and image/svg+xml with the extension and MIME type for the format you actually use:
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// JSON
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['json'] = 'application/json';
return $mimes;
} );
// WebP
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['webp'] = 'image/webp';
return $mimes;
} );Other common mappings include CSV (text/csv), although the correct value can vary by format and server. WordPress’s official hook examples cover SVG, SVGZ, DOC, JSON, CSV, font formats and WebP. Use the MIME type belonging to the actual format, not a guessed value based only on the filename.
How WordPress decides whether an upload is allowed
WordPress builds its list through get_allowed_mime_types(). That process starts with the MIME map from wp_get_mime_types(), removes SWF and EXE mappings, removes HTML and JavaScript mappings for users without the relevant capability, and then applies the upload_mimes filter.
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During validation, wp_check_filetype() compares the filename extension with the permitted patterns and returns the recognized extension and MIME type. A filter therefore adds an allowed mapping; it does not make a renamed or malformed file safe or valid.
When to use mime_types instead
The mime_types filter changes WordPress’s general MIME map. The developer reference describes it as the place to add types globally, while upload_mimes is the practical hook for controlling the upload allowlist (and for removing entries). For a normal “let trusted users upload this one extra format” requirement, use upload_mimes.
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Choose code or a settings plugin
| Approach | Control | Update safety | Operational considerations |
|---|---|---|---|
| Custom plugin or mu-plugin | Precise extension-to-MIME mappings; can be combined with capability checks | High when stored outside a parent theme | Requires PHP deployment and maintenance; works consistently on ordinary single sites |
| Theme code | Can add the same filter | Low for a parent theme; updates may erase it | Use only when the code is in an update-safe child theme and that dependency is intentional |
| Settings plugin | UI-based allowlist, depending on the plugin | Depends on plugin maintenance and compatibility | Useful for non-developers, but still requires correct MIME choices and security review |
For a no-code workflow, WordPress support identifies WPForms File Upload Types as an option for enabling HTML and other extensions. Check the plugin’s current maintenance, compatibility and terms before installing it. A UI does not remove the need to restrict formats and users appropriately.
Security decisions for risky formats
Allow only the smallest set of formats needed for the site. Images such as WebP are generally easier to constrain than active-content formats, but every upload should still be treated as untrusted input.
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- SVG: can contain scripts or other active content. Permit it only for trusted users and consider sanitizing SVG before display.
- HTML and JavaScript: can execute in a visitor’s browser and should be limited to a tightly controlled workflow.
- Archives and executables: can contain executable payloads or dangerous files; do not enable them merely to bypass an error.
- PHP: should not be accepted as an upload format.
Do not solve an “invalid file type” message with define( 'ALLOW_UNFILTERED_UPLOADS', true );. WordPress’s Plugin Handbook warns that this broad switch allows any file type, including PHP and other executables, and creates serious security risks. A specific upload_mimes rule is the least-privilege alternative.
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On WordPress Multisite, check_upload_mimes() intersects the permitted MIME list with the network administrator’s upload_filetypes setting. If the PHP filter appears correct but uploads still fail, a network administrator should open Network Admin → Settings → Upload File Types and add the extension there.
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Capability checks also matter. WordPress treats HTML and JavaScript mappings differently for users who lack the relevant capability, so adding a filter does not necessarily grant every account permission to upload an active format. Keep upload privileges limited to users who need them.
Diagnose “You are not allowed to upload this file type”
- Confirm the filename. Check that the file has the intended extension, has not acquired a second extension such as
.json.txt, and is not merely a renamed file. - Verify the MIME mapping. Match the value to the real format, for example
application/jsonfor JSON orimage/webpfor WebP. - Confirm the code loads. Put it in an active custom plugin or mu-plugin and test an upload after clearing any relevant caches.
- Check the user account. The account may lack the capability required for the requested format, especially for HTML, JavaScript or other active content.
- Check Multisite settings. Ensure the extension is present in Network Admin’s Upload File Types allowlist.
- Check the hosting stack. A host, web application firewall, security plugin or web-server rule can reject an extension even after WordPress accepts its MIME mapping.
If the error changes from a WordPress file-type message to a server or firewall response, the remaining restriction is outside the MIME filter and must be addressed in that layer.
Allow several extensions in one rule
When multiple formats belong to the same site feature, keep them together in one function while retaining explicit mappings:
add_filter( 'upload_mimes', function ( $mimes ) {
$mimes['json'] = 'application/json';
$mimes['webp'] = 'image/webp';
$mimes['svg'] = 'image/svg+xml';
return $mimes;
} );Review the list whenever the feature changes. Removing an unnecessary entry reduces the attack surface and prevents future administrators from assuming that every permitted format is safe for every user.
Quick Recap
Practical decision checklist
- Define the exact extension and its verified MIME type.
- Decide whether the format can carry active content.
- Restrict the rule to the users and workflow that need it.
- Store the code in a custom plugin or mu-plugin rather than a replaceable parent theme.
- For Multisite, obtain network-level approval and update the network allowlist.
- Test a genuine file of the format, then test a wrong or renamed file to confirm validation still behaves as expected.
- Review host, firewall and security-plugin rules if WordPress accepts the mapping but the request is blocked later.
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