This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
To register a file extension as belonging to a specific file type: Choose Tools > Options from the main menu and click Advanced Options in the Options window. In the left pane, expand IDE Configuration > System > Object Types. Select the node of the object type you want to add the file extension to. Select the Extensions and MIME Types property and add your extension to the list.