Note the suspicious lack of commentary on which version of IE supports XMLHttpRequest level 2. Is it supported? And is there an elegant way to fallback to the old iframe approach? Conditionally including Javascript files seems to be a poor choice, I'm not sure if I'd rather just stick with the old way, even if it's awkward.

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An aside: I found the "XMLHttpRequest Level 2" name to be pretty funny. I'm not sure why it tickled my funny bone in such a way.

    A wild XMLHttpRequest (lvl 1) appears!
    Engineer uses SendFile... it doesn't seem to be working.
    Engineer is defeated!

    A wild XMLHttpRequest (lvl 2) appears.
    Engineer uses SendFile... it's super effective!