Send non-images as attachments to force downloads

This fixes a bug in IE where it won't download any attachments at all,
as we previously sent them all as `inline`.

https://www.pivotaltracker.com/story/show/42949101
Merge pull request #246 from alphagov/dynamic-content-disposition-for-attachments

Send non-images as attachments to force downloads

https://www.pivotaltracker.com/story/show/42949101