Thank you for your post.
Unfortunately, I cannot duplicate the error. I have saved files with lowercase, uppercase, mixture of cases, and FileMaker Pro will import the file without incident. FileMaker does not care if the file is named with uppercase or lowercase characters.
When the script fails, does it provide an error message? Does it say the file does not exist? Where is the client saving the file, and is it in the same directory listed in the script? That is what may be causing the problem. Any additional information you can supply could help narrow the possible causes.