In my opinion noun tags should be in singular form unless there is a really good reason to use the plural form.  I would much prefer "buffer" over "buffers", "file" over "files" etc.  Usually when a question applies to *foobars* it also applies to a single *foobar*.

Of course there are always exceptions to a rule: for instance, when tagging questions related to multiple modes then the plural form "multiple-modes" comes naturally and should be preferred.