fix(ui): send JSON body as application/json when no file upload is present (Fixes #17777)

[waterWang]

Summary

Fixes #17777 — The admin panel unconditionally sends all PATCH/POST requests (save, autosave, publish, save draft) as multipart/form-data, even when the request payload contains only JSON data and no file is being uploaded.

Per HTTP semantics, multipart/form-data is intended for requests that include binary file data. When the payload is purely JSON, the request should use application/json. Sending JSON-only data wrapped in multipart encoding triggers false positives in standard WAF configurations (e.g., Cloudflare Enterprise).

Changes

packages/ui/src/forms/Form/index.tsx:

• createFormData: When no file is present in the form data, return the JSON body directly as a string instead of wrapping it in FormData with serialize() from object-to-formdata
• submit: Detect whether the body is FormData (file upload) or a string (JSON), and set Content-Type: application/json header when sending JSON

packages/ui/src/forms/Form/types.ts:

• Updated CreateFormData return type to FormData | string | Promise<FormData | string>
• Updated FormProps action callback type to accept FormData | string

Verified behavior

• Exact same JSON body sent as application/json to the same endpoint: 200 OK
• Same JSON body wrapped in multipart/form-data to the same endpoint: 403 blocked by WAF (Cloudflare Enterprise rule 6179ae15870a4bb7b2d480d4843b323c)
• This fix sends the JSON body as application/json when no file is present, bypassing the WAF false positive
• File uploads continue to use multipart/form-data as before (unaffected)