> use a range or set of icons that change depending on what MIME type the content in that tab is

Most of them will be html or xhtml, though. Hmm... for those, you could do something like: use the domain name as a seed to pick a colour. (either choose from a fairly large set of different-coloured icons, or tint a generic icon)
So all pages on a specific domain should have the same coloured icon, and will be more or less distinguishable from whatever else you're viewing. If a site doesn't want to be mud-brown or pink or whatever they can always make a custom one.