Fix Bad link for download in files

In oc5, 

when you click on the name of the file you go to .../files/download.php?file=XX  and when you click on the action download, you go to  ../files/ajax/download.php?files=XX 

the pbm is that the 1st one does not work because the "file" param of $_GET seems to be already used... so he try to download the file ".php" instead of "XX"

I tried to correct this without touching too much :)