diff -urN saku-3.6.1/shingetsu/thread_cgi.py saku-3.6.1-img/shingetsu/thread_cgi.py
--- saku-3.6.1/shingetsu/thread_cgi.py	2008-10-19 15:34:37.000000000 +0900
+++ saku-3.6.1-img/shingetsu/thread_cgi.py	2009-12-13 21:44:39.000000000 +0900
@@ -32,6 +32,7 @@
 import mimetypes
 import re
 import time
+import imghdr
 from Cookie import SimpleCookie
 from compatible import md5
 
@@ -290,9 +291,13 @@
             self.stdout.write(
                 "Content-Type: " + type + "\n" +
                 "Last-Modified: " + self.rfc822_time(stamp) + "\n" +
-                "Content-Length: " + str(rec.attach_size()) + "\n" +
-                "Content-Disposition: attachment\n" +
-                "\n")
+                "Content-Length: " + str(rec.attach_size()) + "\n")
+            if imghdr.what(attach_file) is not None:
+                #self.stderr.write("print file image type: %s\n" % imghdr.what(attach_file))
+                pass
+            else:
+                self.stdout.write("Content-Disposition: attachment\n")
+            self.stdout.write("\n")
             try:
                 f = file(attach_file, "rb")
                 buf = f.read(1024)