Management Studio won't recognize other file extensions
According to this Connect item, this bug was fixed in Service Pack 1. 

However, if the issue persists, you can try this convoluted workaround posted over at SQLServerCentral by Marios Philippopoulos:


  OK, got it to work with help from http://www.progtown.com/topic374931-sql-server-2012-rtm.html.
  
  
  Opened regedit.exe as Administrator.
  
  
  Then:
  
  
  On [HKEY_CURRENT_USER\Software\Microsoft\SQL Server Management Studio\11.0\FileExtensionMapping\PRC] I changed the Default key value to {B5A506EB-11BE-4782-9A18-21265C2CA0B4}.
  
  
  On [HKEY_CURRENT_USER\Software\Microsoft\SQL Server Management Studio\11.0\Default Editors\PRC] I changed the Custom key value to {B5A506EB-11BE-4782-9A18-21265C2CA0B4}.