post
https://localhost/v4/messages/attachment
Send a message with an attachment. A message can only be sent on behalf of a user if they have the necessary permissions to send
messages in: conversations assigned to them, conversations assigned to others, and unassigned conversations.
Required scope: write_messages.
See our Help Center for a list of supported attachment file types. Please be aware that attachments sent via the API are limited to 30 MB in size and the rate limit is 10 rpm.
Callback