Can you post a bit more of the code that you’re using for this? I can’t see why the system would “see” any difference between an excel file and a text file. As far as I can tell anything in the IFS is just a file to the system, and the extension (.XLS or .TXT) shouldn’t make any difference.
Maybe it’s a difference between the PC applications accessing the files. Maybe Excel really places a lock on the file, whereas Word or Wordpad, etc doesn’t?