When posting FormData via fetch, omit Content-Type so the runtime can attach the multipart boundary

When uploading files via fetch with a FormData body, setting Content-Type: multipart/form-data manually (or via a Headers object that inherits a default JSON Content-Type) drops the auto-generated boundary=... parameter. Servers that parse multipart bodies (FastAPI, Express + multer, Rails, etc.) then return 400/422 because they can't locate the form-part delimiters. The browser, undici (Node 18+ / vitest), and React Native all attach the correct multipart/form-data; boundary=... header automatically when body is a FormData instance and no Content-Type is preset. A shared request<T>() wrapper that defaults Content-Type: application/json for JSON bodies must branch on body type and skip the header set for FormData. Verified 2026-05 against vitest's undici-backed fetch and FastAPI multipart parsing.

In a shared fetch wrapper, only set Content-Type: application/json when the body is a JSON-serialized string. When the body is a FormData instance (or a Blob/File direct upload), leave the Content-Type unset and let the runtime attach the boundary. Verify by reading the resulting request in a test: headers.has("Content-Type") should be false for FormData paths; the server should successfully parse the multipart parts. If a wrapper sets credentials/auth via a default Headers object, branch the Content-Type defaulting on body type rather than always defaulting.