sort matchers by extension makes matching deterministic

[sergolius]

No description provided.

[henvic]

While deterministic is better than undeterministic, shouldn't it try to match to the "most specialized" first? Like, probably change the mimetypes map to a slice of mimetypes and try to sort it manually in the best possible order.

[slockij]

another suggestion - it could also return multiple values (so all matching):
`filetype.MatchMulti([]byte) ([]filetype.Type, error)

[h2non]

This is a good initial approach, but not enough. We need to explicitly order matchers based on the most specific first. I guess it's better to define an array that respects order on each matchers category file.

A PR is very welcome!