LibCore: Implement basic mime type guessing based on binary patterns

This attempts to guess the mime-type from a given set of bytes from the
start of a file. It only supports a few well-defined patterns for now,
but it's a start!

Userland/Libraries/LibCore/MimeData.h

@@ -49,4 +49,6 @@ private:
 
 String guess_mime_type_based_on_filename(const StringView&);
 
+Optional<String> guess_mime_type_based_on_sniffed_bytes(const ReadonlyBytes&);
+
 }