$del_query = "DELETE db_username,filename,filesize,stat FROM uploads WHERE filename = '$delete'";
OOPs! you can't DELETE a field, you can only delete rows!!!
Try this, instead...
$del_query="Update uploads set db_username='', filename='', filesize=0, stat=null FROM uploads WHERE filename='$delete'";
Or, perhaps you meant something more like this?
$del_query="delete from uploads where filename='$delete'";
One of these two.
Hope this helps!