Sorry if this has been addressed in some newer version.
File Roller should use "file" command, or some sort of magic to find out the type of files got as parameters, instead of blindly go after the file extension.

When I try to extract a file with .ipk extension, File Roller says "Archive type not supported", even though the file content is valid .ar format, and if I rename the file to .ar extension, File Roller happily opens it and is able to manipulate it.

The funny thing is that Nautilus is aware of the fact that .ipk extension covers an AR archive, but File Roller is unwilling to handle it as such.