KNL-1278 Only do MIME type detection on first upload of file.

If a file is being updated then we don't do MIME type detection, this allows tools to change MIME types on existing files. The primary reason for this change is to allow people to change the MIME type in Resources -> Edit Details when the detector gets it wrong.

git-svn-id: https://source.sakaiproject.org/svn/kernel/trunk@315470 66ffb92e-73f9-0310-93c1-f5514f145a0a