You are right; the latest 21.2 version of Aspose.Words for .NET API throws File Corrupted Exception upon converting MHTML file.
Message=The document appears to be corrupted and cannot be loaded.
Inner Exception 1:
InvalidOperationException: Root resource is not found in a multipart structure.
We have logged this problem in our issue tracking system. The ID of this issue is WORDSNET-21789. We will further look into the details of this problem and will keep you updated on the status of correction. We apologize for your inconvenience.