I'm having the same issue of 0 byte files appearing on the server. You stated that when you went to an older version of Android it resolved the issue.
I took a look at Telerivet:
I can see it says:
"Telerivet Gateway also supports receiving MMS messages (on Android 4.4 or older)"
Since I assume Jesse reused much of the codebase it seems logical that this is the same version below which the MMS messages work correctly for Envaya also.
I should be able to get a stock ROM for 4.4 or older for the device I have handy, can you please confirm that this was the version you found resolved the issue? Thanks!
Iain