Make an attempt at setting a proper mime-type when showing a file.

Prevents things like having the browser show raw binary data when you have a PDF in your repo.

lib/gollum/frontend/app.rb

@@ -141,6 +141,7 @@ def show_page_or_file(name)
         @content = page.formatted_data
         mustache :page
       elsif file = wiki.file(name)
+        content_type MIME::Types.type_for(name).to_s
         file.raw_data
       else
         @name = name