Support for multipart/form-data Content-Type in HTTP response

[vmaffione]

Hi,
As far as I can see support for multipart/form-data is only available for HTTP requests (both for the client and server side).

Any opinion/plans about adding client and server support also for HTTP responses? This would be handy for all those cases where a HTTP server needs to "atomically" return one or more files (application/octet-stream) along with associated metadata (e.g. JSON, XML), without resorting to encoding the files, e.g. as BASE64 within a JSON field, since this latter solution bloats the response and adds considerable processing and memory overhead...

Thanks

[yhirose]

@vmaffione thanks for the feature request. I don't have time to implement it for now, but I keep it with the 'enhancement' label.

[vmaffione]

Sure, but so do you think it would make sense to implement something like this?
I am still not sure the HTTP standard allows to use such a content-type within a response...

[yhirose]

I haven't investigated if it's possible to implement the feature, yet. Do you know any HTTP libraries which support such a feature? If so, I'll take a look at how those libraries implement it. I personally don't need it on my projects at this point though.

[vmaffione]

To be honest, I only found this stack overflow discussion
https://stackoverflow.com/questions/47067312/multipart-http-response
mentioning that it is possible to achieve that with Node.js

[yhirose]

I am going to close this issue for now.

Question to Copilot:

Is it correct to send 'multipart/form-data' from a HTTP server in a HTTP response? Does HTTP spec allow it?

Copilot says:

Yes, the HTTP specification does allow a server to send a multipart/form-data response, but it's not common and should be used with care.

What the HTTP Spec Says:

The HTTP/1.1 specification (RFC 7231) and related MIME specifications (like RFC 2046) allow multipart content types in both requests and responses. So technically, a server can send a Content-Type: multipart/form-data in a response.

However, multipart/form-data is primarily designed for HTTP requests, especially for file uploads via HTML forms. For responses, other multipart types are more common, such as:

• multipart/mixed
• multipart/byteranges (used in range requests)
• multipart/alternative

Why It's Rare in Responses:

1. Browser Support: Most browsers and HTTP clients are not designed to handle multipart/form-data in responses. They expect it in requests.
2. Semantics: multipart/form-data is specifically structured for form submissions, with field names and file content. That structure doesn't usually make sense in a response.
3. Better Alternatives: If you're trying to return multiple files or data parts, multipart/mixed or a zip archive is often more appropriate.

When It Might Be Okay:

If you're building a custom client-server system (not relying on browsers), and you control both ends, you can use multipart/form-data in responses. Just make sure your client can parse it correctly.