2050  wget http://ftp.digium.com/pub/asterisk/webmin/webmin.tgz
 2051  tar xfz webmin.tgz 
 2052  tar xfvz webmin.tgz 
 2053  cd webmin
 2054  tar cfz webmin_work.tgz webmin_work/
 2055  mv webmin_work.tgz ..

I did the above and the used "From uploaded file" to upload and install webmin_work.tar.gz

Bottom line.  That's a tar of the source for the module, not just the installer.

I don't have asterisk on the machine I loaded it on, but it did install after the above...