I followed this link and applied the HTML mime type and worked fine.
This are the instructions from the webpage:
Go to Netbeans -> Preferences -> Miscellaneous -> Files on a Mac
or Tools -> Options -> Miscellaneous -> Files on Windows
Leave "File Extensions" as "< Choose the extension >" and click the "New" button.
Type in the extension without the dot.
Select the appropriate file type from the "Associated file type (MIME)" drop down box.
http://www.electrictoolbox.com/netbeans-syntax-highlighting-file-extensions/