diff --git a/src/main/scala/lc/RainDropsCaptcha.scala b/src/main/scala/lc/RainDropsCaptcha.scala
index 0b79add..089f607 100644
--- a/src/main/scala/lc/RainDropsCaptcha.scala
+++ b/src/main/scala/lc/RainDropsCaptcha.scala
@@ -116,7 +116,7 @@ class RainDropsCP extends ChallengeProvider {
     ios.close
 
     // ImageIO.write(canvas,"png",baos);
-    new Challenge(baos.toByteArray, "image/png", secret)
+    new Challenge(baos.toByteArray, "image/gif", secret)
   }
   def checkAnswer(secret: String, answer: String): Boolean = {
     secret == answer