Sorry for my late answer.
There is this plugin: buddypress-message-attachment
But it is outdated.
So, if you can, for the next rtMedia update, includes this function.
I can see that rtCamp has this function.
For free version. Files: Jpg, Jpeg, Png, PDF, Mp4, Mp3 (adjust, optional from rtSettings)
Max: 5MB only (min – optional '')
Pro Version: Files: All files (optional '')
Max: more then 6MB (optional '')
From the web (optional '')