[BUGFIX] Wrong extension used for the default table icon

The default icon for a database table was referencing the extension „hdnet“ which isn't necessarily availabe.
Switching the default icon to use the extension icon from „autoloader“ and adjusting for the PNG icon type being used.

Change-Id: I975c175e5b6f349004826685c50008ba22eecb00
Resolves: #60422
Reviewed-on: http://review.typo3.org/31870
Reviewed-by: Tim Lochmüller <tim@fruit-lab.de>
Tested-by: Tim Lochmüller <tim@fruit-lab.de>