when clicking on an attached visio file on the attachment page, it will not open embedded with visio viewer 2013, see attached screenshot.
It works with visio viewer 2010.
And it works on Confluence 3.5 with visio viewer 2013.

When you delete the last part of the url from the attachment (?api=v2) then IE shows the visio file embedded with viewer 2013.
Url in Confluence 4.3.7: http://confluence.com:8080/download/attachments/32309801/example.vsd?api=v2 --> works only with viewer 2010
Url in Confluence 3.5.11: http://confluence.com:8080/download/attachments/32309801/example.vsd works with viewer 2010 and 2013