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JSON.stringify() turns a JavaScript value into JSON text; it does not upload a file’s bytes. If you place a browser File object in a JSON payload, the request will not contain the file in the usual multipart upload format. The file has not been deleted from the user’s device—the request is simply using a body format that does not match what the upload endpoint expects.
Why the file goes missing
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 send its content using request-body formats designed for files. JSON is for structured text data, such as metadata or an identifier; stringifying a file object does not turn its bytes into a conventional file upload.
For more on the serialization behavior, see MDN’s JSON.stringify() reference. The browser’s File documentation and guide to using files in web applications describe how selected files are exposed to JavaScript.
Choose the request body the endpoint expects
| Request body | Use it when | Implementation point |
|---|---|---|
FormData |
The endpoint expects a multipart form, often with a file and ordinary text fields. | Append the file using the field name the server expects. Let the browser set the multipart content type and boundary. |
Raw File or Blob |
The endpoint expects one binary body directly. | Pass the file as the body and follow the endpoint’s method and content-type requirements. |
| JSON string | You are sending structured metadata, identifiers, or upload-session details. | JSON text is not a multipart upload of the selected file’s bytes. |
Fetch accepts FormData as well as File and Blob request bodies. The right choice depends on the receiving endpoint’s contract, not on which format is easiest to stringify. See MDN’s FormData guide and the MDN Fetch API guide.
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Upload a file as multipart form data
For an endpoint that expects a standard multipart form upload, append the selected file to a FormData object and pass that object directly as the Fetch body:
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,
});- Read the selected file from
input.files[0]. - Append it under the field name the server expects. In this example, that name is
file. - Send the
FormDataobject as the request body. - Check the server response and confirm the endpoint accepts the field name and file size.
Do not manually set Content-Type: multipart/form-data for this request. The browser must add the multipart boundary that separates the form parts; setting the header yourself prevents it from supplying the required boundary expression.
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When a raw file body is the right choice
If the endpoint is designed to receive one binary body rather than multipart fields, pass the File or Blob directly as the Fetch body. Set the request method and any required content type according to the server’s contract. This is not interchangeable with multipart: a server expecting form fields may reject a raw body, just as an endpoint expecting raw bytes may not parse a multipart request.
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Why is my file still not uploading?
- Check the field name. A file appended as
filewill not satisfy an endpoint expecting a different form field. - Check the body format. Use
FormDatafor a multipart endpoint, or a rawFile/Blobonly when the endpoint accepts a binary body. - Check the request headers. When sending
FormData, do not set the multipartContent-Typemanually. - Check the response and upload limits. A client-side format change cannot fix a server rejection, a file-size limit, or a mismatch with the endpoint’s contract.
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