Use wand :original_filename override

When the attachment is a temporary file from an upload it will
not have a file extension more than likely. This means that MIME
cannot determine the file and we fall down to 'file' command more
than we should have to.

requires raykrueger/wand@4268216

lib/joint.rb

@@ -44,7 +44,7 @@ def #{name}=(file)
           else
             self["#{name}_id"]             = BSON::ObjectId.new if self["#{name}_id"].nil?
             self["#{name}_size"]           = File.size(file)
-            self["#{name}_type"]           = Wand.wave(file.path)
+            self["#{name}_type"]           = Wand.wave(file.path, :original_filename => Joint.file_name(file))
             self["#{name}_name"]           = Joint.file_name(file)
             assigned_attachments[:#{name}] = file
             nil_attachments.delete(:#{name})