Please describe the problem:
Currently, file-roller depends on the filename extensions for some archive
formats, for example gzip or bzip2 and doesn't open a file if extension is
missing or different :(
This is very reminiscent of the broken Windows paradigm. file-roller should
determine the file type based on content rather than name.

But file-roller can open some other archive types, for example zip archives
without extensions or with other extension, for exampe .rar without problems.

Other software, for example nautilus or file command, always detect archive type
correctly, even if extension is missing or unknown, for example .dar or .jigdo

Steps to reproduce:
1. Compress some file, for example
http://cvs.gnome.org/viewcvs/*checkout*/file-roller/ChangeLog to ChangeLog.gz
2. Remove .gz extension or replace .gz extension to some other, for exampe
rename to ChangeLog or ChangeLog.jigdo
3. Try to open renamed file (from nautilus or from File-Roller's
Archive->Open... menu ) 


Actual results:
File-Roller doesn't open renamed archive file and displays an error message:

Could not open "ChangeLog.jigdo"

Archive type not supported.

Expected results:
File-Roller should open renamed archive file, because other GNOME software, for
example Nautilus or file command detect file type correctly:

ubuntu@ubuntu:~/Desktop$ file ChangeLog.jigdo
ChangeLog.jigdo: gzip compressed data, was "ChangeLog", from Unix

Does this happen every time?
Yes

Other information:
Same problem is with bzip2 and possible other types (I tested only 3 types -
gzip, bzip2 and zip), while file command detect file type correctly:

ubuntu@ubuntu:~/Desktop$ file ChangeLog2.dar
ChangeLog2.dar: bzip2 compressed data, block size = 900k