You can add handlebars.coffee to the fileTypes array in HTML.tmLanguage. After I did this, Sublime opened the sample handlebars.coffee file as html syntax. Here are the relevant lines from my HTML.tmLanguage file:
<key>fileTypes</key> <array> <string>handlebars.coffee</string> <string>html</string> <string>htm</string> etc. </array>
You can access this file by opening the HTML folder in settings> view packages ...
Be sure to save the new version of this file in the user's preferences folder. This will save it through updates.
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