Don't arbitrarily reject content types in form post attachments.
Attachments to forum posts are accepted or rejected on the basis of content type (various image and documents accepted; mp3, mp4, etc. rejected). The excuse given is about file size. A single JPEG image can easily be a larger file than many minutes of MP3 audio, yet up to 20 files of 20MB each are accepted of the approved content type.
Content type is not a valid proxy for file size.
As musicians, small audio clips are our lingua franca, and rejecting these from forum postings drastically devalues the forum as a tool.