back out fix for bug 243833 since it caused regressions

mail/test/mozmill/attachment/test-attachment.js

@@ -97,13 +97,6 @@ var setupModule = function (module) {
                       filename: 'ubik.xxyyzz',
                       format: '' }],
     },
-    // cert attachment (make sure Mime doesn't swallow this, which
-    // test_attachment_view_expanded does for us.
-    { attachments: [{ body: binaryAttachment,
-                      contentType: 'application/pkcs7-mime',
-                      filename: 'cert.p7m',
-                      format: '' }],
-    },
   ];
 
   for (let i = 0; i < messages.length; i++)

mailnews/mime/src/mimei.cpp

@@ -765,8 +765,7 @@ mime_find_class (const char *content_type, MimeHeaders *hdrs,
           if (name) {
             char *suf = PL_strrchr(name, '.');
             if (suf &&
-                (!PL_strcasecmp(suf, ".p7c") || !PL_strcasecmp(suf, ".p7z") ||
-                 !PL_strcasecmp(suf, ".p7m")))
+                (!PL_strcasecmp(suf, ".p7c") || !PL_strcasecmp(suf, ".p7z")))
               clazz = (MimeObjectClass *)&mimeExternalObjectClass;
           }
           PR_Free(name);