From time to time you may see that an email has an untitled attachment or ATT00001.htm or ATT00001.txt or even Winmail.dat
This is produce when the sending composing mail client , configuration is set to send as 'Rich Text format' and also on occasions via HTML It is completely normal behavior for sending mail clients sending in Rich Text format' to produce this type of attachment file by default when sending in this way, even if the original email has no attachment that the sender has applied.
For these reasons if a sender sends via RTF (rich test format) and you have inserted the .htm extension as an extension to block within your Content Control, we will interrogate the email at a binary level and capture the email correctly.
End users or recipients may not even see that the original email has such an attachment as it may not be visible from within the recipients mail client and will appear as though the email has no attachment. We will capture the email correctly with the reason unknown.002
The Winmail.dat for instance is a file is used to preserve Rich Text formatting as an attachment and Outlook uses it when sending a Rich Text-formatted message.