Parse a multipart/mixed or multipart/form-data response body.
Handles preamble (content before first boundary) and epilogue (content after final boundary) by skipping them appropriately per RFC 2046.
The fetch Response with Content-Type: multipart/*.
An array of parsed MultipartPart (headers + body).
If no boundary is found in Content-Type.
Parse a multipart/mixed or multipart/form-data response body.
Handles preamble (content before first boundary) and epilogue (content after final boundary) by skipping them appropriately per RFC 2046.