Hello Jason.

Media gallery web part can be used to display content of media libraries only. So for files stored in given media library, in your file system. If you want to show attachments of some custom document type (unsorted or grouped), you can use other web parts.

You can find example for unsorted attachments and web parts you can use here, for grouped attachments please read this.

In case of need, please clarify. Thank you.

Best Regards,
Radek Macalik