sendToChat allow to specify mime type

[singpolyma]

It would be nice if https://webxdc.org/docs/spec/sendToChat.html could accept a mime type. Right now we must infer type only from filename extension if present.

[r10s]

well, in my experience, mime types do add confusion only, not clarity :) the file extension is usually unique enough, i do know of very few examples where this is not the case, and none of them come into mind when working with webxdc. but maybe i have overseen a larger real-world usecase :)

anyways, iirc, you already now can set a mime type when using a File or a Blob object, maybe this is good enough and we need only explicit documentation upon that