Validate multipart/form-data end-to-ends

[bterlson]

Multipart/form-data endpoints are super common, but we don't have much coverage for them at this point and it's likely things are missing or non-optimal (#2370 is an example).

At a minimum, we need to support being explicit about the headers for each part, content-type and content-disposition in particular.

We may also want to consider going beyond OpenAPI and being explicit and treating filename and name as explicit parameters/return types so we can express whether these are optional or required.

[bterlson]

Another thing to consider: allowing clients to pass a custom boundary in cases where autodetection is unreliable or undesirable.

[bterlson]

How to specify multiple content-type application/json parts.

[markcowl]

• sample
• docs
• cadl-ranch test
• openapi emitter support (if any)

est: 2

design issues:

• client specified boundary
• handling file names / file name required
• how to differentiate arrays where each element is a part from an array that is meant as an array within a single part?

[mikekistler]

FYI: Playground link

[qiaozha]

Another thing to consider: allowing clients to pass a custom boundary in cases where autodetection is unreliable or undesirable.

Should boundary be a part of the content type of multipart/form-data like Content-Type: multipart/form-data; boundary=abcde12345 and typespec compiler should not generate constant content-type for multipart requests ?

Another question is, I feel like content-type and content-disposition are some kind of things each languages' core libraries should support during serialization and deserilization ? Though we probably need the information about the content-type of each parts.

how to differentiate arrays where each element is a part from a part that contains an array

Kind of lost in this part, does this mean the body is an array of mixed multipart type ?

[timotheeguerin]

@qiaozha I think custom boundary by the user is not something we have designed yet so I would hold off on any assumption of what the user could do to make it work. Same for question 2 this is still pending some design.

For the last question if we are working with part that are basically an array how do we know this is meant to be an array of part vs just a regular array (more design needed)

[qiaozha]

Thanks @timotheeguerin for sharing this.

Just a little curious, is it a blocker for openai non branding work ? should we support custom boundary and content-type for different parts in openai's case ?

[markcowl]

Complete