Creates an upload middleware factory.

Checks the declared Content-Length, reads the body once through ctx.request.bytes() and caps that length against resolveMaxBodyBytes BEFORE the form is touched, then obtains the form through the request's formData() accessor — or, when the request omits that optional member (an out-of-repo IRequest), through the same shared parseFormBody the accessor itself calls — and enforces maxSize/allowedMimeTypes/maxFiles on the field's FILE parts before storing the result under 'storage-plugin:uploads' in ctx.state. Every refusal short-circuits without calling next. Parsing is the shared one parse (M94b): a csrfFormMiddleware that already read the same form ahead of this middleware costs it nothing, because the accessor memoizes. The policy (every bound and every refusal status) stayed here; only the parse moved to common.

A part carrying no filename under the field name is a plain form value in the web standard's terms and is no longer reported as an upload — see the CHANGELOG migration note.

Refusals are answered 413 when something was too large — the request body against resolveMaxBodyBytes, or one file against maxSize — and 400 when the request was genuinely malformed or otherwise unacceptable (too many files, a disallowed MIME type, an unparseable body). Both size refusals previously answered 400, which told a client it had sent something malformed when it had only sent something big.