Added Filemanager

There's no way you can see the files that you have uploaded in OGV. Every-time you wish to see a model you have to upload it again. I have created a simple file manager that would show the files added by the user as tiles and clicking on them would open the respective file in viewer. For that I have made some changes to upload_file.php so that it takes in account whether the request is coming from file_manager.php.