[Do not merge] Add icons for common attachment types

With the introduction of supplementary data attachments, I think different file type attachments will become more common (particularly spreadsheets)

I still have some kinks to work out, but I need some advice.

Here's what this looks like so far http://imgur.com/SQBw4pe

First of all, am I doing all of this correctly?

Then there are a couple more specific issues:
1. The database is not updated for current installations. I'm sure there's a way to do it, I just don't know how (yet).
2. Snapshots are identified as HTML attachments. Technically they are, but I think we probably want to distinguish them. Although thinking about this now, the only translator that I'm aware of that attaches HTML documents is Google Scholar and it does so using `snapshot: true`. So this issue is probably quite involved. Perhaps in the mean time I will just use snapshot for webpage items.

Icons are not yet correctly shown in the pop-up dialog. I'll work on that.