This fixes a couple bugs that have snuck in when commenting on legacy

screenshots. The first one was that the screenshot comment model doesn't define

canUpdateBounds, which was causing an error trying to call it. While fixing

that I fixed up some documentation on the matching version for image file

attachments. The second bug was that the screenshot model doesn't have a

filename attribute, which was causing a similar error when trying to decide

the default zoom level to use.

Was able to comment on a screenshot without errors. Verified, to my surprise,

that the new zoom feature worked without issues.