Just in case anybody else stumbles upon this topic because the server cannot parse your multipart/form request:
The OpenAI API currently cannot handle boundaries that include dashes (-). If you construct your request manually or the web client you use inserts dashes as part of the boundary, you might need to redefine these boundaries. I think this is a bug in the API since RFC1341 explicitly permits this:
boundary := 0*69<bchars> bcharsnospace bchars := bcharsnospace / " " bcharsnospace := DIGIT / ALPHA / "'" / "(" / ")" / "+" / "_" / "," / "-" / "." / "/" / ":" / "=" / "?"
https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html#:~:text=bcharsnospace