Don't convert MIME content-type headers to lower case as this seems to cause problems.

Thanks to xingwang.xu for the analysis.

Fixes issue 1289

src/com/fsck/k9/mail/internet/MimeBodyPart.java

@@ -100,7 +100,7 @@ public class MimeBodyPart extends BodyPart
         }
         else
         {
-            return contentType.toLowerCase();
+            return contentType;
         }
     }