This is a general "problem" with the split files. The video itself does not have the .txt extension. The big files are split using Hacha or some other similar software. The extension given is something like .0, .1, .2, etc. or .001, .002, .003, etc. Those extensions are not valid for attachments in messages. Valid extensions are .txt or .zip, among others. The easiest thing to do, it to ZIP all the split files and post them after they are compressed. People downloading the attachments just need to download them and unzip them. After they are unzipped, then you join them with Hacha or another software.
However, some people go through the pain of renaming all the files to have "dummy" .txt or .zip extensions. The people downlaoding the attachments then have to remove the dummy extension and join them. This does not always work (not sure exactly why) as you get "Invalid file" errors if you try to join them after removing the extensions. I think this workflow is more prone to human error instead of just zipping them for real.