Improve illegal 'Content-Transfer-Encoding' error

index.js

@@ -213,20 +213,17 @@ MIME.decodeBody = function(buffer, contentType, contentTransferEncoding) {
     // Further, some systems incorrectly apply a content-transfer-encoding to a
     // multipart body, causing corruption and/or missing multiparts if the
     // encoding is decoded.
-
-    // Options:
-    // 1. Reject content-transfer-encoding on all composite media types.
-    // 2. Reject content-transfer-encoding on multiparts.
-    // 3. Reject content-transfer-encoding on multiparts but allow base64.
-    // 4. Ignore content-transfer-encoding on multiparts.
-    // 5. Follow content-transfer-encoding on multiparts.
     if (
-      /^(message|multipart)\//.test(contentType.value) &&
       contentTransferEncoding !== '7bit' &&
       contentTransferEncoding !== '8bit' &&
       contentTransferEncoding !== 'binary'
     ) {
-      throw new Error(self.Error.ContentTransferEncodingComposite);
+      if (/^message\//.test(contentType.value)) {
+        throw new Error(self.Error.ContentTransferEncodingMessage);
+      }
+      if (/^multipart\//.test(contentType.value)) {
+        throw new Error(self.Error.ContentTransferEncodingMultipart);
+      }
     }
     if (contentTransferEncoding === 'base64') {
       // RFC 2045 6.8 Base64 Content-Transfer-Encoding
@@ -3390,8 +3387,11 @@ MIME.Error = {
     "(see RFC 2045 2.2, RFC 2046 4.1.2, RFC 2047 3 and RFC 2231).\r\n",
   CommentUnterminated: "550 Your email had a header with an unterminated " +
     "comment (see RFC 5322 3.2.2).\r\n",
-  ContentTransferEncodingComposite: "550 Your email had an illegal " +
-    "'Content-Transfer-Encoding' mechanism for a composite or 'multipart' " +
+  ContentTransferEncodingMessage: "550 Your email had an illegal " +
+    "'Content-Transfer-Encoding' mechanism on a 'message' composite media " +
+    "type (see RFC 2045 6.4).\r\n",
+  ContentTransferEncodingMultipart: "550 Your email had an illegal " +
+    "'Content-Transfer-Encoding' mechanism on a 'multipart' composite media " +
     "type (see RFC 2045 6.4).\r\n",
   ContentTransferEncodingUnrecognized: "550 Your email had an unrecognized " +
     "'Content-Transfer-Encoding' mechanism (see RFC 2045 6.1 and 6.4).\r\n",