Nautilus offers the option to change the default app with which to open a *type* of document, but there is no way to get it to open one variant of a type of document, differentiated by suffix, and leave the others alone.
eg I want to open all files with ending .lea with leafpad, but continue to open all other "plain text document" files with the default editor.
When I go to set the option to set leafpad.lea to open with leafpad, nautilus will only give me the option to "open leafpad.lea and all other files of type "plain text document"" with leafpad.
This is not necessarily a bug, but it seems like a fairly serious shortcoming.
More info
If I make a new text document and call it kwrite.kwr, all the same problems apply, but if I touch kwrite.kwr on the command line, and then set the resulting file to open with kwrite, the suffix kwr is then opened by default by kwrite. This would suggest that the functionality proposed here is readily available, and 'just' needs implementation - provided it is seen as desirable. It is also this latter behaviour that suggest that there might be something like a bug, since this proceedure does not work for the suffix .lea - though presumably this means that .kwr is already in the system in some way, and .lea is not.

ProblemType: Bug
Architecture: i386
DistroRelease: Ubuntu 9.04
ExecutablePath: /usr/bin/nautilus
Package: nautilus 1:2.26.2-0ubuntu2
ProcEnviron:
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: nautilus
Uname: Linux 2.6.28-15-generic i686