It's the other way around: the extension check is ran before the classifier. And given the limitations of file extensions, an extension collision is so likely of an occurrence (not most likely, but likely enough) as to not bother with primary_extension and just use the collection of extensions as a culling system to minimize the work for classification.

In other words, it seems like the overall design wouldn't be hurt too much by just extricating primary_extension completely. Best-case scenario, primary_extension is equivalent to always having at least one item in extensions. It does nothing else.

Also, this looks like a bug: "if possible_languages.length > 1 ... else possible_languages.first" What if length is 0? That's not greater than 1. I'm not familiar with Ruby, does first return null on an empty array, or does it error? LINQ in .NET has separate First<T> and FirstOrDefault<T> methods: one errors, the other returns default(T) (which is null in the case of reference types). Or is there a default match in the index that occurs when no other language is found? https://github.com/github/linguist/blob/master/lib/linguist/...

Not instilling a lot of confidence that someone really thought through this bit of code. I'm not saying I very strictly think through everything I write, but I also don't write software for thousands of users, and I acknowledge that I've grown rather complacent in terms of time spent per unit code.