Copied from the bug tracker on SourceForge:
https://sourceforge.net/tracker/index.php?
func=detail&aid=1400574&group_id=72728&atid=535427

See also: http://code.google.com/p/ganttproject/issues/detail?id=113

1. Start GP (optionally open/create some project)
2. Open HTML export tool (Project->Export->HTML report)
3. Open the file chooser dialog, type file name without
extension (e.g. "foobar")
4. Press OK. 

Result: extension is not added automatically and links
between pages of the generated report are broken

Expected: extension should be automatically added,
links should not be broken