[KingMob]

You can technically add a Content-Length header for each part. It's not forbidden by the RFC, but nor is it common. It caused problems (https://github.com/square/okhttp/issues/2138) for OkHttp, and they eventually removed it. Might be fine for internal-only use, though.

Boundaries are a lot like UUIDs, and rely on the same logic. When generating random data, once you have enough bits, the odds are against that sequence of bits ever having been generated before in the universe.

[KingMob]

EDIT: Was looking at the obsolete RFC. The current version, RFC 7578, actually forbids all part headers other than "Content-Type, Content-Disposition, and (in limited circumstances) Content-Transfer-Encoding".