use grit to get the file's mime type, as suggested.

http://github.com/github/gollum/commit/6de9ddfd3f79a9cb320aab3c43b8e0a912107b2e#commitcomment-137656

lib/gollum/file.rb

@@ -33,6 +33,11 @@ module Gollum
     # Public: The String path of the file.
     attr_reader :path
 
+    # Public: The String mime type of the file.
+    def mime_type
+      @blob.mime_type
+    end
+
     #########################################################################
     #
     # Internal Methods