filtering file selection widget

[Stephen Dunn <stephend cogs susx ac uk>]

Hello

	I'm using the file selection widget to choose files which must
only have a certain extension from a directory. Is there anyway of
getting the file selection widget to only display those files ending
with a certain extension?

For example: In a directory I have the following files:

myfile1.txt
myfile2.html
myfile.c 
myfile2.txt

and I wish the user to only be able to select from the two files
ending in ".txt".
How might I do this in gtk?

Many thanks,
Steve