The ATT00001.txt file is a file generated as an attachment by Microsoft Outlook or Microsoft Exchange Server. The name ATT00001 is a generic file name and you’ll often see these in .htm format, .dat, and other extensions when there is some sort of compatibility issue between the sender and recipients Microsoft email program settings.
It is often the result of a MIME file being sent and because of a compatibility issue when the redundant / additional txt file gets generated and attached. These Microsoft mail systems use Transport Neutral Encapsulation Format (TNEF) to pack and send this information, which is a proprietary format that sometimes leads to problems.
Was this added as a garbage attachment and has no content in it?
This file type is typically going to either be the plain text of the email or blank. If its blank, you will need to check with the sender, as the attachment didn’t send properly and was replaced with this garbage/blank file.