[ticket/11912] Improve extension map in content_guesser

PHPBB3-11912
[ŧicket/11912] Get rid of obsolete variables in content_guesser

PHPBB3-11912
[ticket/11912] Remove typo in content_guesser

PHPBB3-11912
[ticket/11912] Supply filename to content_guesser for guessing on windows

The filename of the files sent to the guesser by plupload do not contain
the file extension. Therefore, it's impossible to guess the mimetype if
only the content_guesser is available and the function mime_content_type()
doesn't exist. By supplying the filename we can circumvent this issue.

PHPBB3-11912
[ticket/11912] Introduce guesser_interface

This will contain proper documentation of the required methods if anyone
would implement a newer guesser.

PHPBB3-11912