I've been working this issue with Atlassian Support.
If I access any issue that has a document attachment, and click on the attachment link, nothing happens. If instead, I click on the icon of the attachment the attachment downloads. This behavior only seems to be broken for attachments that do not open in an inline modal. E.g. jpg attachments work with both the icon and the URL, but docx attachments only work when clicking the icon and do not work when clicking the URL.
The issue is cross browser for us, occurring in Chrome v49.0.2623.87 m, Chrome v49.0.2623.108 m, and Firefox v45.0.1 on Windows 7 and Firefox v31.5.0 on Linux. Clearing browser cache/cookies does not make any difference.
We were able to isolate it to a problem with the attachment URL and not also the attachment thumbnail by stepping through the click event Handlers in a Chrome debugger session. It was just dumb luck that allowed us to determine the behavior difference between images and documents.
We performed the following tests:
clicked link on a document (docx) attachment - nothing happens
clicked thumbnail on a document (docx) attachment - attachment downloads
clicked link on an image(png) document attachment - image opens in modal dialog
clicked thumbnail on an image (png) attachment - image opens in modal dialog
I discovered that if I disable the emailThis plugin, the problem went away and if I re-enabled the emailThis plugin the problem came back. There are 66 modules within the emailThis plugin. If I disable the specific plugin module, emailissue-web-resource6, the problem goes away. If I re-enable this specific plugin module, the problem comes back.
SLES 11 SP3 64-bit