You currently can only add files with the extension .html. Yes, of course you can just add .html to a .txt file, but it is not the same, the hash changes. It would be nice to be able to include .txt as an extension.
+1 Although you cannot do this right now, you can reference already uploaded files on the server so it's doable, just not as convenient as it could be.
There are a lot of file types that need to be importable that aren't at this time. One of the most major ones are the new SVG image standards as well as the ever lasting PDF files along with the TXT and many others I'm sure. Hopefully we'll see this eventually.