Don't place asset files or yaml files in layouts

Ref: #2402

lib/jekyll/document.rb

@@ -88,6 +88,14 @@ module Jekyll
       !(asset_file? || yaml_file?)
     end
 
+    # Determine whether the file should be placed into layouts.
+    #
+    # Returns false if the document is either an asset file or a yaml file,
+    #   true otherwise.
+    def place_in_layout?
+      !(asset_file? || yaml_file?)
+    end
+
     # The URL template where the document would be accessible.
     #
     # Returns the URL template for the document.

lib/jekyll/renderer.rb

@@ -47,11 +47,15 @@ module Jekyll
         output = render_liquid(output, payload, info)
       end
 
-      place_in_layouts(
-        convert(output),
-        payload,
-        info
-      )
+      if document.place_in_layout?
+        place_in_layouts(
+          convert(output),
+          payload,
+          info
+        )
+      else
+        convert(output)
+      end
     end
 
     # Convert the given content using the converters which match this renderer's document.