Example of image name copy-pasted into Confluence from the clipboard:

 image2013-1-11 19:17:13.png 

The format is:

 image<YYYY-M-DD><space><HH:MM:SS>.extension 

See this screen cast for sample: http://www.screenr.com/YQK7

Confluence has this bug: https://jira.atlassian.com/browse/CONF-26131 where System Error is encountered when clicking "Download All" if attachment name contains some characters. One of the character that is causing System Error is double colon ":"

As long as Confluence do not discard the time formatting HH:MM:SS in the attachment name, Windows OS users will automatically be affected with this bug. Renaming the file to discard the colon one-by-one via Properties is not feasible.

Ultimately, fixing the bug will fix this issue.