Fixed 0008589: file_download.php doesn't work with attachment names that contain accentuated characters when using Firefox.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4759 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9