PHP Warning: move_uploaded_file() unable to move

This solved the problem for me:

$ sudo chown -R www-data:www-data /var/www/html/

Change upload permissions for /var/www/media2net/uploads/ either by changing owner with "chown" or by "chmod"

Examples

$ sudo chown apache:apache /var/www/media2net/uploads/
$ sudo chmod 755 /var/www/media2net/uploads/

Also, if downloaded_file.png already exists in that directory and it's owned by another user, then you would need to change ownership on that file as well.

$ sudo chown apache:apache /var/www/media2net/uploads/downloaded_file.png

This way, it can be successfully overwritten by Apache.