No New Extensions showing
I have tested the same against civicrm 4.7.12 against Drupal and wordpress  - I do get the extension list - so I dont think this is to do with civicrm 4.7.12 - 

May be please check if you have dropped the extension in the correct location also make sure your directory path is point to the correct location- 

If nothing works try to clear your cache and try again 

I guess this helps!!!