This is an Inbox+ feature, which is currently in Open Beta. Please note that this feature is in active development and functionality or specific directions outlined in this FAQ may change.
You can send an image or document to your users through Inbox in Optix.
Navigate to the chat of your interest in the Inbox Plus section.
Click on the blue (+) icon OR drag and drop the document into the chat
Send the message and the file will be shared directly with the user.