Can't attach random-access file

[SMcNeill]

It's plain text, saved without an extension. Maybe I need to add a .txt extension?

That's the whole issue in a nutshell.  Web traffic has various rules for transferring data properly, so just anything you ever share on the net is going to need some sort of extension to identify it.   In this case, you could've just added that .txt extension and it would've worked fine.  Without an extension, it doesn't know how to deal with things (is it a sound to play, or a video, or text, or an image, or what??), so whenever it comes to almost anything you share on the web it'll need that extension for properly storing and processing it. 

(Or need to be inside a zip/7z/tar/rar type file, which it knows how to handle due to their extensions.  )