Improve UX of displaying attachments

[dotmacro]

Problem

When a user adds a file to a comment, the file does not always show at all.

1. Image attachments show the file name as plaintext (and the broken image icon, depending on browser)
2. Non-image attachments (e.g. .mov) show nothing, appearing as a blank comment

Solution

1. Image attachments should display as images, thumbnails, and/or a way to easily view the image
2. Non-image attachments should display as thumbnails, file icons, and/or a way to easily view or download the attachments