Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsIf a browser upload sends a FormData object, pass it as the request body and do not set the Content-Type header yourself. The browser must generate a multipart boundary and use the same value in the header and body. A 400 response alone does not prove that a boundary is the problem, so inspect the outgoing request and confirm what the endpoint expects before changing code.
Why multipart requests need a boundary
A multipart/form-data request carries separate parts—such as text fields and files—in one body. The parts are separated by a boundary string. The request header declares that string, for example Content-Type: multipart/form-data; boundary=ExampleBoundaryString, and the body uses matching delimiters such as --ExampleBoundaryString. The receiver needs that match to find where each part starts and ends. RFC 7578 defines this format; MDN’s Content-Type reference also documents the boundary parameter.
If the header says only multipart/form-data, or its boundary differs from the delimiters in the body, the server may be unable to parse the form. Each part also needs the expected field name; RFC 7578 specifies a Content-Disposition header with a name parameter for each part.
Fix browser Fetch and XMLHttpRequest uploads
When the body is a browser FormData instance, let Fetch or XMLHttpRequest set the multipart Content-Type header. Manually adding Content-Type: multipart/form-data can prevent the browser from adding the boundary parameter, even though it serializes the body with boundary delimiters. MDN’s FormData guidance explicitly says not to set this header for FormData requests.
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Fetch
const form = new FormData();
form.append("file", fileInput.files[0]);
form.append("description", "Example upload");
const response = await fetch("/api/upload", {
method: "POST",
body: form
});Do not add a Content-Type entry to headers. The browser will construct the header and body delimiters together.
XMLHttpRequest
const form = new FormData();
form.append("file", fileInput.files[0]);
const xhr = new XMLHttpRequest();
xhr.open("POST", "/api/upload");
xhr.send(form);Likewise, do not call setRequestHeader("Content-Type", "multipart/form-data") for this FormData body.
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Axios: check the runtime and payload type
Axios’s guidance differs by environment and FormData implementation. Its multipart documentation says browser, web worker, and React Native FormData should be sent without explicitly setting Content-Type; let the runtime add the boundary. In Node.js, Axios documents using the form-data library and copying its headers when the object provides getHeaders(). Do not apply a browser fix blindly to Node or assume every Axios adapter handles every payload identically.
| Request path | Boundary handling | What to check |
|---|---|---|
| Browser Fetch or XMLHttpRequest with FormData | The browser generates the header and matching body delimiters. | Leave Content-Type unset; send the FormData instance as the body. |
| Axios with browser, web worker, or React Native FormData | The runtime should add the boundary according to Axios guidance. | Do not explicitly set Content-Type; verify the actual runtime, Axios version, and adapter. |
Axios in Node.js with the form-data library |
The library can provide headers through getHeaders(); Axios documents copying them. |
Confirm the FormData implementation and resulting headers rather than using browser assumptions. |
| Postman Body > form-data | Postman automatically attaches Content-Type, unless a manually selected header takes precedence. | Check for a stale or manually entered Content-Type override. |
A narrowly scoped report on Axios issue 7054, opened September 27, 2025, describes Axios 1.12.0 with the fetch adapter omitting a boundary when Content-Type was manually set to multipart/form-data. Treat that as a report about the stated configuration—not proof of behavior across Axios versions or adapters. Reproduce the request using the version, adapter, and FormData implementation in your application.
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Reproduce the request in Postman
- Open the request and choose Body > form-data.
- Add a row for each field. For file data, set the field type to File and select the file.
- Send the request and inspect the outgoing headers and body if the endpoint still returns 400.
- In the Headers tab, look for a manually selected Content-Type. Postman documentation says its generated Content-Type for form-data can be overridden by a manually selected header.
Postman’s form-data documentation describes this request setup. A successful Postman request can help isolate request construction, but it does not establish that your production client sends the same headers or body.
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Verify the actual request before changing server code
- Capture the outgoing request. Use browser developer tools, a proxy, or the API client’s console. Record the method, endpoint, Content-Type including parameters, and whether the body is actually multipart. Source-code configuration alone may not show the final request after client transformations.
- Check for the boundary parameter. If the Content-Type is multipart but has no
boundary=..., the receiver lacks the declared delimiter value. - Compare header and body. Confirm that the boundary parameter exactly matches the body delimiters, including the closing delimiter. RFC 7578 describes delimiters formed from CRLF, two hyphens, and the boundary value; the boundary must not occur inside an encapsulated part.
- Apply the client-specific fix. For browser FormData, remove the manual Content-Type header. For Axios, verify runtime, adapter, version, and FormData implementation, then follow that environment’s guidance. In Postman, inspect manual header overrides.
- Confirm endpoint expectations. Check that the endpoint expects
multipart/form-data, and that field and file names match what it requires. If it expects JSON or URL-encoded data instead, changing the boundary will not fix the format mismatch. - Investigate other validation failures if multipart is correct. If the header and body boundary match, inspect the response body and server logs for endpoint validation, missing required fields, file limits, authentication, or other application-specific causes.
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