Well I went back and re-downloaded the Fairy Tail and extracted it with 7z 9.25 and I didn't get an error. I uninstalled 7z 9.20.  So it looks like for people who have 7z 9.20 would have to update to 9.25 and it should work no problem but you may get an occasional failure if so then re-download and try again. And to be on the safe side of things check the CRC with Rapid CRC or Anime Checker.

anyway, i got reply from anonfiles saying  "Please try now".
i think they will now allow .mkv just thought to let you know :)

Urgh all of them do give an error for me, and my guess is that they're random bit flips :/

[IB] Mondaiji-tachi ga Isekai kara Kuru Sou Desu yo - 05 [720p] [10bit] [D1705E86].mkv
Archive file: 1360501961576.7z (MD5: 009ec4697ab8c3cd07eb38cbb4a5d2e4)
Byte 0x1fc5faf is 0x7c (1111100 in binary) but should be 0x78 (1111000 in binary)
Byte 0x49d93ab is 0x6d (1101101) but should be 0x69 (1101001)
Byte 0xab13baf is 0x58 (1011000) but should be 0x59 (1011001)
(there's more for this file)

[CureCom] DokiDoki! Precure - 01 [720p][F943F56C].mkv
Archive file: 1360503954441.7z (MD5: 41ebe438edef70eb4dbde94bd5b3539e)
Byte 0x11e239af is 0x5d (1011101 in binary), but should be 0x59 (1011001 in binary)

Once you make the above single bit changes they extract fine.
The thing that needs to be figured out is whether the corruption comes from my script or file transfer. Because these are all single bit flips, my instinct is with transmission, or something at AnonFiles, in which case, I probably can't do much about it.

- Update -
Okay, there definitely is a problem with AnonFiles. I just redownloaded 1360501961576.7z ([IB] Mondaiji-tachi ...) and got an MD5 of 8a795b6514beec9b2d748124148ff606
In simpler terms, this means that AnonFiles is corrupting the data sent back randomly.
I'll do some further investigation on this a bit later.

- Update 2 -
I think some of the MD5s I stuffed up above, but anyway, just downloaded 1360501961576.7z a few times, and AnonFiles seems to be spitting out different files each time! (none of which extract without error)

MD5: 8a795b6514beec9b2d748124148ff606
MD5: 652eaa2da052530d01ea797969f3a8a7
MD5: ed48e4d50b74860a3da17ffb85944e33
MD5: 90fe55ba0446b673a8d1f24935d2f703

I may look into seeing how they're stuffing up later, but I'll likely remove AF soon because of these random corruptions.

I may consider doing byte swap encoding rather than delta coding, where single bit flips won't cause the entire file to corrupt (though you'll still get CRC errors).
Thanks for the examples.

Unfortunately, even if .mkv is allowed, there's a number of problems:
a) files that exceed 500MB will need to be split (and thus won't have a .mkv extension)
b) they seem to be doing a header check, so files that don't have a proper MKV header, even if the extension is .mkv may be rejected - I suppose these are rare, but I don't want to deal with such a possibility
c) non-MKV files may still need to be wrapped

So for simplicity's sake, I'm going to wrap everything in a .7z
The only real solution is if they remove the file type check altogether.

Thanks for trying nevertheless :)

i sent email to anonfiles to allow .mkv extension