Can't disassociate a file type in RSA 8.5

I am unable to disassociate a file type in Rational Software Architect 8.5.

Here is what I did:
Using Window->Preferences->Team->Jazz Source Control->File Properties I selected "Text->Perl Source File."
I added the file type *.tt to that list.  That changed all of the *.tt files to Perl file types.

I now want to disassociate *.tt from Perl.

I tried going to Window->Preferences->Team->Jazz Source Control->File Propertie->Text->Perl Source File and clicking the"Remove" button.  This removed the *.tt from the list. 

When I create a new *.tt file, it is now categorized as a plain file, but existing *.tt files haven't changed.

I also tried adding *.tt to the Text list, where *.txt is, but that didn't have any effect, either.

The one way I can seem to get around this is by copying the *.tt file, pasting it, removing the original, and then renaming the copied file.  This is obviously not the ideal workaround.

Has anyone ever seen this before?  Is there another way to do this, or a better workaround?

Thanks in advance for any help.

-Amy