This is my use case: the page I'm testing returns CSV file. SimpleTest tries to parse it and runs out of memory (btw, my memory limit is 1 GB :). It may be sound idea not to parse non-HTML (maybe XML too) content, just represent it as a large string, so text assertions will still work, since they may be useful still---for example I'm checking for PHP errors regexp, even in CSV output.
Sorry if it's not a bug, but feature request---I can re-submit it into proper tracker.
Assigning this to lastcraft, since I dupe'ed the other bug.
Also setting bug up in an appropriate category.
Relate to the bug: I think checking the MIME type to see if the page is HTML should be sufficient; we shouldn't engage in Internet Explorer style sniffing.