Add spec for image/vnd.microsoft.icon ico file

spec/lib/gitlab/utils/mime_type_spec.rb

@@ -24,6 +24,7 @@ RSpec.describe Gitlab::Utils::MimeType do
         "rails_sample.png"           | "image/png"
         "rails_sample.bmp"           | "image/bmp"
         "rails_sample.tif"           | "image/tiff"
+        "sample.ico"                 | "image/vnd.microsoft.icon"
         "blockquote_fence_before.md" | "text/plain"
         "csv_empty.csv"              | "application/x-empty"
       end