MP3 Files cannot be played or downloaded in the mobile app.
Mattermost Server 5.19.1
Mattermost iOS & Android App 1.27.1
Mattermost Desktop App 4.3.2
System Console > File Sharing and Downloads> Allow File Downloads on Mobile enabled
Send an MP3 file as an attachment in public / private channel / direct message.
Tap on the file to play or download it.
Users are allowed to download the file into the mobile device.
The file appears as an image preview instead.