diff --git a/lib/attachments_controller_patch.rb b/lib/attachments_controller_patch.rb
index f5240be..0737f73 100644
--- a/lib/attachments_controller_patch.rb
+++ b/lib/attachments_controller_patch.rb
@@ -8,7 +8,7 @@ module AttachmentsControllerPatch
               @diff = File.new(@attachment.diskfile, "rb").read
               @diff_type = params[:type] || User.current.pref[:diff_type] || 'inline'
               @diff_type = 'inline' unless %w(inline sbs).include?(@diff_type)
-              if (@diff.start_with?("From"))
+              if (@diff.start_with?("From") and @diff.match(/(Subject.*)^---$/m))
                 @diff_format_patch = @diff.match(/(Subject.*)^---$/m)[0].sub(/^---$/m, '').force_encoding('UTF-8');
               else
                 @diff_format_patch = nil;