Adds missing use LWP::MediaTypes statement for guess_media_type

The refactoring of the attachment processing code in the REST interface
in commit 7b6ddfb did not include the use statement for
guess_media_type. The consequence being if you post attachments without
content-type, RT errors and includes a Perl traceback in the API
response.
Remove unnecessary "use LWP::MediaTypes" lines

The code which needed them was removed in 7b6ddfb
Merge branch '4.0/add-missing-use-lwp-mediatypes' into 4.0-trunk