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
This commit is a highly-refactored and up-to-date version of Fyorl's work
which was part of his Google Summer of Code 2012 project "Attachment
Improvements".
PHPBB3-10929
