This is one of those "I swear I'm not making this up!" situations.
We choose to have voicemail messages from our university phone system sent to our helpdesk email as a .wav attachment. In the past, prior to the implementation of the oAuth stuff in OS Ticket, this worked just fine. A customer would leave a voicemail for us, campus voicemail system routes it to our inbox, which OS Ticket then picks up. All is well. This is what it used to look like:

Ever since our IT staff implemented oAuth for us in OS Ticket, we've been experiencing a bizarre problem: the email notice from our voicemail system comes through like it always has, but the attached .wav file is stripped out completely.

I've tested everything I can think of, such as composing a message to our helpdesk and attaching a .wav file in case all .wav's are being blocked or something.. that comes through fine. I have OS Ticket logs set to debug, and haven't found anything interesting at all. It's literally just voicemail messages attached from our voicemail system... so strange!
Can anybody suggest some troubleshooting tips?
Jim