ByExtension calls mime.TypeByExtension, and removes optional parameters, to keep only the type and subtype.

FromReader takes a reader, sniffs the beginning of it, and returns the mime-type (no charset) and a new reader that's the concatenation of the bytes sniffed and the remaining reader.

Match returns the mime-type (no charset) if it can guess from the first bytes, or the default content-type else.

DefaultType is the type used when we can't know/guess the filetype.