Arbitrary Method Invocation

This commit addresses possible Arbitrary Method Invocation via AJAX file upload.

To save some memory osTicket uses callback method to fetch the content of a
file on mail fetch. $file['data'] was overloaded as a callback by simply
checking if the content is callable, resulting in method invocation when
content of the uploaded file is a callable.

The address the issue we're not using locally set callback parameter / method.