Using the built-in Android Browser or Chrome on Android Devices, i recognized that all downloaded files are renamed to "downloadfile.<ext>". (while <ext> is set correctly according to the mime_type of the file)
I figured out that this is affected by a bug in the Android Download Manager which doesn't react in the correct manner. According to the Information provided in the above link, I looked at how the headers are set in
includes\process-download.php. line 69 creates an output similar to this:
Content-Disposition: attachment; filename="MyFileName.ZIP";
...which will lead the Android Download Manager to ignore the filename provided, because of the trailing semicolon.
Removing the semicolon fixes the issue.
A short test on a Desktop Browser (current Chrome) after the change showed normal behaviour.