GetOpenFilename, .xls And .txt

I was wondering if there is a way to show just Excel files AND text files, specifically files with the extension .xls and .txt.

Part of my program converts a text file to an excel file, I would like the user to be able to see if the .xls file has been created and if so chose whether to open the .txt file or the existing .xls file. If the above isn't possible, can I check if a .xls file exists in a folder, if I did this I could then get the user to select which type of files they would like shown.

I know I could just have no filtering of the file type at all, which would work, but that wouldn't look as good, and I'm all about how the user interface looks!

Perfect, thanks