Symptoms

A file has its proper extension within fisheye. For example fisheye displays the file name example.rmp.
If you right click on the link to download and choose "copy link location" and paste this location to a new browser window, you can see it is the correct link for the example.rmp file, e.g. it looks like http://localhost:8060/browse/~raw,r=1.4/path/to/example.rmp.
However when you try and download the file via IE, the file is downloaded as example.zip. The file won't open, because its not a zip, hence you have to rename the file back to example.rmp before you can open it correctly.

If you use Firefox, then the file downloads as example.rmp file as expected.

Cause

This looks like a bug within IE, as IE for some reasons interprets certain files as .zip.

Other IE users have had similar problems, see this and this page.

Resolution

User Firefox when attempting to download these files, or when you are saving the file, rename to the proper extension.