Atom includes a feature called "custom file types" which you can use by adding some entries into your config.cson that look like this:
core:
customFileTypes:
'source.ruby': [
'Cheffile'
'this-is-also-ruby'
]
'source.cpp': [
'h'
]
The key (for example source.ruby in the above snippet) is the language's scope name. The value is an array of file extensions, without the period, to match to that scope name.