To return non-HTML responses (like XLS spreadsheets), we usually use the

respond_to do |format|
  format.xls do
    # send spreadsheet
  end
end

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This is often, but not always the same as checking for params[:format] == :xls, so don't rely on this when e.g. one format checks for authorization and the other doesn't.

params[:format] is only set when a user explicitly puts a .xls at the end of the URL.

The format.xls block also responds when the user's browser requests the application/excel MIME type.

If Internet Explorer runs on a machine with Excel installed on it, this is done automatically.