files with extension 'lha' should handeld as lha archives. 
i have patched fr-archive.c to fix this.

this is the diff:  
root@debian:/usr/src/file-roller-2.6.1# diff -u src/fr-archive.c.orig
src/fr-archive.c
--- src/fr-archive.c.orig       2004-10-07 21:47:54.000000000 +0200
+++ src/fr-archive.c    2004-10-07 21:31:52.000000000 +0200
@@ -397,7 +397,8 @@
        } else if (file_extension_is (filename, ".zoo")) {
                archive->command = fr_command_zoo_new (archive->process,
                                                       filename);
-       } else if (file_extension_is (filename, ".lzh")) {
+       } else if (file_extension_is (filename, ".lzh")
+                  || file_extension_is (filename, ".lha")) {
                archive->command = fr_command_lha_new (archive->process,
                                                       filename);
        } else if (file_extension_is (filename, ".rar")) {
root@debian:/usr/src/file-roller-2.6.1#