filename limitation for file download phprunner 5.1 2135

I have created a number of online databases using PHPrunner 5.1 2135 and notice the following:

If I create a field for file upload, the file can be uploaded and it appears in the list, but depending on

1. length of the filename (sometimes), or arrangement of hypens in the filename, users are unable
   
   to download the file. With different browsers and an affected filename, you get different errors.
   
   Firefox will just hang, Opera gives an error that says the file doesn't exist, IE who knows.
   If I re upload the same file with a new short filename, it can be downloaded normally.
   At first I thought it was the length of the filename (40 plus characters) that was causing the problem, but I have some 36
   
   character filenames that will not download either while 38 character filenames will download normally.
   Regards,
   
   Bill