Implement multipart file uploads

[abonander]

I couldn't find anywhere documenting a desire or otherwise a lack of desire for multipart/form-data support.

I have a very basic streaming multipart client API in multipart-async now: https://github.com/abonander/multipart-async/tree/master/src/client

It's not quite production ready yet though. I want to add several more tests. It's also built against tokio-io instead of futures-io. Whoops.

[yoshuawuyts]

@abonander yeah, having this would be great! -- I considered implementing it before the initial release, but this is somewhat tricky and we'd need to come up with a good design first. Would you like to work on bringing this into Tide?

[abonander]

@yoshuawuyts Tide or Surf? I also have a server API that's just about ready for use which would close http-rs/tide#89. I mostly just want to do a bunch more testing to really be confident in it but it's already better tested than multipart ever was since I have the benefit of hindsight.

I think the client API design is comparatively straightforward, though. The main question is whether it should be lazy (pass a list of fields and the client processes them in a single call at the end) or eager (actively write each field to the transport). We have a much better error handling story in Rust now than when I developed the original multipart client API (which the lazy API was meant to paper over so every call didn't need to be wrapped in try!()) so I think just an eager API might be fine.

[futurist]

What's the plan of this feature? It's an important feature I think (or maybe bug?)

[Fishrock123]

See http-rs/http-types#175 - likely to picked back up soon over there, unsure how or when that will impact a full surf api for this.

[Fishrock123]

I don't have any investment in this anymore. Someone else will need to pick it up.