See related bug CONF-2487. It seems that the very same problem is still valid when an attachment has a file name containing special characters (like ä,ö,ü) and spaces. In this case when downloading the attachement file, all spaces are replaced by "+" signs. If documents have no special characters, the spaces are preserved on download - as expected.

This is problematic, especially because if this document having "+" instead of spaces is then edited and re-uploaded to the wiki page, it is not recognized as new version of the original document but as a new document because of the different file name.

I could not find a hint that this has been fixed/altered in the bug fix issus for 4.3.x so I assume this is still present in current 4.3.6 version.
(Althoug CONF-22338 has been fixed in the mean time.)

Note that this issue seems Firefox specific (like CONF-12671), with IE there is no such problem.