Accessing Data stored in an iframe

If I have a .txt file embedded into my webpage via an iframe, how do I access the data stored in it? I had assumed that accessing the innerHTML would do fine, but since its not HTML, it doesn't work.

Is there an equivalent accessing scheme for other kinds of files other than HTML? Code currently looks like this:

innerDoc = document.getElementById("state_current").contentWindow.document;
var state = innerDoc.innerHTML;

via Chebli Mohamed