Display custom field under image in TinyMCE editor

I have a custom media field called “Source” with the key media_author. I would like to display it under the image in the TinyMCE editor, like the caption field is:

The caption field is editable, but that’s not necessary for what I want to add. I found this code in /wp-includes/js/tinymce/plugins/wpeditimage/plugin.js. It looks like it’s part of how the caption is displayed.