How did you make sure that the problem is related to the regular expression supplied by tobyink?

Most likely, this is another error in your script that is totally unrelated to the regular expression.

Maybe you should eliminate all other code and only keep the code that relates to the "convert file window", whatever that is. I don't see your code trying to load or save a file in the block governed by the regular expression, so you are either not showing the relevant code or are misinterpreting how your program behaves.