MHTML document do not have a specific binary signature. However, as a secondary verification, I put in a NoFileTypeKnownAs field its equivalent as per its extension. This type of document comes in two varieties. Sometimes, it is MHT and sometimes it is MHTML. Both are returned by Firefox as MHTML document. Between the two extensions, if I would have to allow the download of such time from a Web site to a user, generating a temporary file name, would it be better to use .mht or .mhtml?