(#2701) - fix blob content type in Chrome 37

OK, so here's what I'm doing:

Chrome 37 claims to have blob support, but really it's just messed up, because they might return a null content type for PNG images and maybe other types of binary data (literally the string `"null"` instead of `"image/png"`.

So my fix is to augment the blob support detection by inserting an image, and then doing an ajax request to make sure that the content type of that image is actually `'image/png'`.

Interestingly, Firefox doesn't let you do ajax requests on blob URLs. They just throw a 405. IE10 allows it, though.