BTW, if someone wasn't aware of this yet, direct URL to image actually contains MD5. I just wasn't certain that what versions MD5 this is because most of the time it didn't match any version of the image.
media.tumblr.com/<RAW MD5>/tumblr_<string>_<quality>.png

So if the _1280 is identical to raw, then that part in URL should match the filename on e6.

But take this also with grain of salt, as I have only tested this on handful of posts and didn't seem to be case with couple GIF animations at all.