mmm: conversations et al seem to use a 16 bit IV instead of the 12 bit IV from
the spec, even the example in the spec uses a 16 bit IV... this makes the total
size of the hex string in the anchor 96 bytes instead of 88 bytes

spec: https://xmpp.org/extensions/inbox/omemo-media-sharing.html#aesgcm

example from the spec

GET URL: https://download.montague.tld/4a771ac1-f0b2-4a4a-9700-f2a26fa2bb67/tr%C3%A8s%20cool.jpg
IV: 8c3d050e9386ec173861778f
Key: 68e9af38a97aaf82faa4063b4d0878a61261534410c8a84331eaac851759f587ed40ca58
Resulting URL: aesgcm://download.montague.tld/4a771ac1-f0b2-4a4a-9700-f2a26fa2bb67/tr%C3%A8s%20cool.jpg#
8c3d050e9386ec173861778f68e9af38a97aaf82faa4063b4d0878a61261534410c8a84331eaac851759f587ed40ca58

Own example with actual encrypted data:

aesgcm://upload.snel.it:5281/upload/ti4zbdGcBegishjp/zg99zm1TS1y44J01Q-G1iQ.jpg#84492c0395fdd6e3da05e2b936c7a6fdf0ed770f42862044f3eadf9c2f4d0d48d998f5eeec69ae0bb63497f97c0ed06a

anchor: 84492c0395fdd6e3da05e2b936c7a6fdf0ed770f42862044f3eadf9c2f4d0d48d998f5eeec69ae0bb63497f97c0ed06a

link: https://upload.snel.it:5281/upload/ti4zbdGcBegishjp/zg99zm1TS1y44J01Q-G1iQ.jpg

iv: 84492c03 95fdd6e3 da05e2b9 36c7a6fd

key: f0ed770f 42862044 f3eadf9c 2f4d0d48 d998f5ee ec69ae0b b63497f9 7c0ed06a