[ticket/11148] Add mimetype guesser to filespec and fileupload class

The mimetype guesser will be used to get the mimetype of uploaded files.
Until now, this was only used for files uploaded with plupload. If a file
doesn't have a mimetype supplied, we will now try to get the correct mimetype.

PHPBB3-11148
[ticket/11148] Pass mimetype guesser to upload_attachment() function

PHPBB3-11148
[ticket/11148] Use mimetype guesser for uploaded avatars

PHPBB3-11148
[ticket/11148] Default to application/octet-stream if no mimetype given

This should prevent us from having an empty mimetype while uploading a file
using local_upload().

PHPBB3-11148
[ticket/11148] Add missing parts to docblock of get_mimetype() method

PHPBB3-11148
[ticket/11148] Get rid of extra line in mimetype guesser setter doc block

PHPBB3-11148
[ticket/11148] Always use the output of the mimetype guesser in get_mimetype

PHPBB3-11148
[ticket/11148] Change expected output with disallowed content in test

PHPBB3-11148
[ticket/11148] Remove unneeded variable mimetype and use type octet-stream

The variable $mimetype is not used in the method local_upload() afterwards
so it shouldn't be assigned. The correct default mimetype should be
application/octet-stream and not application/octetstream according to RFC 2046.

PHPBB3-11148
[ticket/12051] Optimise Composer Autoloader on Build

PHPBB3-12051
[ticket/12847] Allow the extensions to say if they can be enabled

PHPBB3-12847
[ticket/12693] Add a travis test that checks file permissions

PHPBB3-12693
[ticket/12693] Fix indentation

PHPBB3-12693
[ticket/12693] Check if the files have the right 644

And not only if they are executable.

PHPBB3-12693
[ticket/12693] Fix the permissions

PHPBB3-12693
[ticket/12693] Check if the are executable and not if they are 644

PHPBB3-12693
[ticket/12693] Check the persmissions of the owner

PHPBB3-12693
[ticket/12693] Remove the executable bit on icon_print.gif

PHPBB3-12693
[ticket/12693] Extract exceptions, bin/* must be executable

PHPBB3-12693
[ticket/12693] Fix the query for bin/*

PHPBB3-12693