Overhaul content-type handling, making it configurable via page/proxy commands as well as frontmatter with the 'content_type' parameter.

Now, users can set content type explicitly for their files in a number of ways, or rely on automatic file-extension content types. Proxied files default to automatic file-extension content types, but then fall back to the content type of the resource they proxy to. There is also a bug fixed around correctly setting content type inside send_file. Fixes #821.

middleman-core/lib/middleman-core/core_extensions/front_matter.rb

@@ -185,6 +185,15 @@ module Middleman::CoreExtensions
       def data
         ::Middleman::Util.recursively_enhance(raw_data).freeze
       end
+
+      # Override Resource#content_type to take into account frontmatter
+      def content_type
+        # Allow setting content type in frontmatter too
+        fm_type = data[:content_type]
+        return fm_type if fm_type
+
+        super
+      end
     end
 
     module InstanceMethods