[ticket/11912] Add extension_guesser for guessing mimetype by extension

The content_guesser now only guesses the mimetype with the function
mime_content_type() while the guessing by extension is done using the
extension_guesser.

PHPBB3-11912
[ticket/11912] Introduce guesser priority to mimetype guessers

The mimetype guesser priority can now be set through the service definition.
Mimetypes will be guessed from the guesser with the highest priority to
the one with the lowest priority. Standard priority types have been added
to the service definition file. Any integer value can be used though.
Standard mimetype guessers that do not have the methods get_priority
and set_priority implemented, like the standard MimeTypeGuessers of symfony,
will have the default priority with the value of 0. Lower priority guessers
have values lower than 0 while high priority ones can be added with values
higher than 0.

PHPBB3-11912