Adding mime_types to 'Create a new gallery'

I’ve been staging a personal foray into the media modal and custom gallery templates for a plugin i’m developing. Most all is going well. Using the filter 'ajax_query_attachment_args' found in wp-admin/include/ajax-actions.php, I have successfully allowed all mime_types to appear in the media modal when ‘Create Gallery’ is selected. However, upon selecting the files, and clicking ‘Create a new gallery’, they are not added. I can however, add them by clicking ‘Add to Gallery’, selecting them a second time, and submitting. Any thoughts on how I can make it work on the first go? Been trying to wade through core files, but have come up unsuccessful thus far.