Attachment display mods

Hi Boone-

This is not a bug fix request; this one's totally a matter of presentation. I've replaced the thin paperclip icon with one matching the other icons. It looks good to me at full-desktop size, but may be a little too bold at a tablet size. I'll have to borrow a tablet to check it out. If this is close, but not perfect, please give me some feedback and I'll make modifications. If this isn't something you want to do with bp-docs core, I can do most of it with css, so no problem. I can do the other by adding a filter on bp_docs_attachment_item_markup().

Thanks,

-David