Thank you!
Here is the source code for send_message method: https://github.com/mailboxer/mailboxer/blob/b53bab9e93dafcfa499205f2e3170c2b8acb0064/lib/mailboxer/models/messageable.rb#L65
And here is the uploader: https://github.com/mailboxer/mailboxer/blob/1618ae474fa49d6cd583838fbaea827f21fe79cb/app/models/mailboxer/message.rb#L14
So basically they use carrierwave to manage that stuff https://github.com/carrierwaveuploader/carrierwave/