I've had the same issue since the upgrade to 1.17 with Cisco Unity voicemails (.wav files). With prior versions, the wav attachment would be imported in OSTicket properly when fetched over IMAP. After the upgrade and switching to OAuth2 for authentication with Office365, the email is still discovered by OSTicket but the attachment is not imported.
I am curious you found a solution @ellisonrje , @geseronta?
Environment details:

Are you getting a message in the ticket thread about the attachment?
Nothing shows up indicating that there was an attachment or error
What are you using as a storage backend? (ie where are attachments stored)
Database storage, 10MB limit and working well for any other type of attachments
Did you list .wav as an acceptable attachment type?
Yes, we also tried without any attachment type restriction. Other email attachments are working well. Attaching the same voicemail file in a regular email works fine and is imported. Cisco Unity uses the Unified Mailbox to push emails in a mailbox (rather than sending to a mailbox).
I will send the headers to support[at]osticket.com.