JSON.stringify() turns a JavaScript value into JSON text; it does not upload a file or delete it from your device. The usual problem is that a selected File object has been placed in a JSON payload, which does not send the file bytes in the multipart format an upload endpoint expects. For a standard form upload, send the file with FormData instead.
Why the file seems to disappear
A file input exposes selected files through its files property, a FileList containing File objects. A File is a kind of Blob, and browser APIs can use its contents in supported request-body formats. But converting an object to JSON is not the same as sending its binary contents as a file upload. The server may receive metadata or an unhelpful representation instead of the file bytes it expects.
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The JSON.stringify() reference describes converting JavaScript values to JSON text. For details on browser files, see MDN’s File reference and guide to using files from web applications.
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How to send a file with fetch()
Use FormData for a normal multipart upload
When the endpoint expects a file together with ordinary form fields, create a FormData object, append the selected file using the field name the server expects, and pass the object as the Fetch request body:
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const input = document.querySelector('input[type="file"]');
const formData = new FormData();
formData.append("file", input.files[0]);
const response = await fetch("/upload", {
method: "POST",
body: formData,
});In this example, file is the multipart field name. Replace it with the name defined by your endpoint’s contract. The browser builds the multipart body and supplies its boundary, which separates the parts. Do not manually set the request’s Content-Type to multipart/form-data: doing so can prevent the browser from adding the boundary the server needs to parse the upload. MDN explains this behavior in Using FormData Objects.
For multiple selected files, append each File under the field name or names required by the endpoint. Check the server response and its upload limits; a correct client request cannot fix a mismatched field name or a server-side rejection.
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Send a raw File or Blob when the endpoint expects one binary body
Fetch also accepts a File or Blob directly as the request body. Use this when the receiving endpoint is designed to accept a raw file body rather than multipart fields. Set the method and any required content type according to that endpoint’s contract. The Fetch API documentation describes supported request bodies in Using the Fetch API.
Choose the request body the endpoint expects
| Request body | Suitable when | Implementation point |
|---|---|---|
FormData |
The upload includes a file and ordinary text fields in a multipart form. | Append the file under the expected field name; let the browser set the content type and boundary. |
Raw File or Blob |
The endpoint accepts one binary body directly. | Use the request method and any content type required by the server. |
| JSON string | You are sending structured metadata, identifiers, or an upload-session detail. | JSON is not a substitute for sending the selected file as a multipart upload. |
Keep JSON for metadata, not the file transfer
JSON is useful for structured information such as metadata, identifiers, or details about an upload session. Send that information in the format the endpoint specifies. When the endpoint needs the file itself, use its supported file-body format rather than assuming that placing a File in an object and calling JSON.stringify() sends the bytes.
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