Content-type method missing for Tempfile


I have created a group model that has a picture. On uploading the
picture the variable uploaded_file is set as the Tempfile uploaded. I
then have an after_save method that creates several thumbnails of this
picture using RMagick. The trouble I am having is when I try and copy
a group with some pictures. I would like to be able to make another
Tempfile which was the original picture file and set it to the
variable uploaded_file. Then when save is called on this copied or
"cloned" group it too will save thumbnails of the main picture.

The trouble is that when I create the Tempfile that is the same as the
original file. For some reason the method 'content-type' which is
available after an upload is missing. Also, I can't seem to find the
method anywhere in the Rdoc for a Tempfile.

Can someone help me please? Any ideas?