MediaType isn't extensible

[mredig]

I am currently working on an HLS server and was working to serve the stream components through HB, however when I load an m3u8 file through FileMiddleware, there's no definition for m3u8 in MediaType so the browser just attempts to load it as plain text.

Obviously, the simplest solution would be to just add m3u8 to MediaType.extensionMap, but I think that's just a band aid. Devs should be able to define additional MediaTypes that can be used by FileMiddleware.

I would propose that MediaType.extensionMap be made privately mutable and to add a static registerAdditionalType(_ type: MediaType, withExtension ext: String) to MediaType (or, if scoping is preferable for some reason I can't foresee, perhaps registering on an instance of FileMiddleware)

I'd be happy to make a PR for this if you guys are onboard.

[adam-fowler]

I would propose that MediaType.extensionMap be made privately mutable and to add a static registerAdditionalType(_ type: MediaType, withExtension ext: String) to MediaType (or, if scoping is preferable for some reason I can't foresee, perhaps registering on an instance of FileMiddleware)

Making MediaType.extensionMap privately mutable will have Sendability issues. It might be better to add an extension, media type map to FileMiddleware.

[mredig]

What would be the downside of wrapping access to the property in a lock, to maintain Sendability safety?

[mredig]

It might be better to add an extension, media type map to FileMiddleware.

If we do this, I assume we'd want to leave extensionMap on MediaType and just copy it as a starting place on FileMiddleware instances?

[mredig]

I created a PR with the latter idea. Let me know if you have any suggestions or would prefer a different approach.