>>/4451/
On your OP:
> application/x-7z-compressed

> application/x-7z-compressed,

you have 7z twice, one without a comma: it maybe why .7z files are not working.
Just tested it with this file: RWtBR.7z https://my.mixtape.moe/kuehqn.7z 8.11 MB (8,508,300 bytes)
---------------------------
Checksum information
---------------------------
Files: 1
Folders: 0
Size: 8508300 bytes
CRC checksum for data: CA90C4CF
CRC checksum for data and names: FE1EF805
SHA-256: 21449C4C1D0F96C907B846499238B761C32D73B28829AE5F9480E7F26AA00BF2
---------------------------
OK   
---------------------------

Tried to upload it on:  >>/v/1175/

Tor Browser Bundle: 6.0.2 (based on Mozilla Firefox 45.2.0)
Format: 7z
Compression: Ultra:
Method: LZMA
Dictionary: 64MB
Word size: 64
Solid Block Size: 4GB
2 Threads

http://mime.ritey.com/ returned nothing, even with JS enabled. You can replicate if you like.

about:config
noscript.allowedMimeRegExp default string ""

Anything else you need?