multipart/form-data
The standard HTTP form for file uploads — each field separated by a "boundary" string, binary data encoded raw.
RFC 7578. Header: Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryXYZ. The body consists of blocks separated by --boundary, each with its own headers (Content-Disposition, optionally Content-Type). Required for <input type="file">. URL-encoded doesn't work for binary data. Modern alternative: chunked POST with a JSON body + base64, or signed URL directly to S3.