Could you please open a terminal and type:
file /path/to/your/file/filename
(e.g. file /home/mercylotus/.aMule/Incoming/file.rar)
The output should be something like
file.rar: RAR archive data, v1d, os: Win32
If it isn't you've maybe downloaded a fake file, i.e. someone gave any file the name 'file.rar' even if it is something completely different.
Regards