It looks like the combination of Windows 2008, DotNetNuke 4.8.3, and the forums are creating a situation where attachments causes this forum to crash.
I did find where the problem was, and I believe that I fixed it. Could you please download the file below and test it?
TextFileSplitter_1.3_debug.zip
Once you test it and it fixes your issues, I will post it to the public.