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You're contriving a bunch of make-up reasons. You aren't making any kind of a "case" by making the filenames look worse than they are (pasting URL-encoded text instead of the actual filenames to make it look ugly; using obviously contrived examples like a tagme and a rare tag-heavy post). A typical filename is "moe 27550 blanc_neige elwing shining_tears tony_taka.jpg", which is nice and clean.

Download filenames are for humans, and MD5s as filenames are opaque and useless to humans. I can't find anything in a directory full of them. It's interesting that you mention sorting, since these filenames usefully sort by post number, where MD5s sort statistically randomly.

Long filenames are cut down to 150 characters or less.

If you want to have MD5 filenames, you can write a GM script to rewrite "http://moe.imouto.org/image/MD5/text.jpg" URLs to "http://moe.imouto.org/image/MD5.jpg".