How to force a true TXT extension?

[William de Smet]

Hi there,

Been busy for a while with my little program and ran into the  
following problem:
The user can save a file with a .TXT extension and with Windows XP  
the dialog box shows the suggested *.TXT as filename.
When deleting or overwriting the suggested name (without extension)  
the saved file is not recognized as a .TXT file.

How can I force a true .TXT extension (even when deleting the  
suggested name)?

I use the following code:
on mouseUp
ask file "Bewaar woordenlijst als:" with "*.TXT"
   put field "woorden_uit" into URL ("file:" & it)
   answer "De woordenlijst is bewaard!"
end mouseUp

Thanks!

William de Smet