IconVault's FormData Inspector lets you assemble a FormData object field by field, text fields and file uploads alike, then reveals the exact multipart/form-data body it would send: boundaries, Content-Disposition headers, filenames and Content-Type per part. Completely free and runs fully in your browser.

See an entries table with types and sizes, the estimated request size, and the raw wire-format body you can download. It is the fastest way to debug file uploads, check what your fetch call really sends, or learn the multipart format hands on.