Windows Explored

Every now and then you might encounter a message that contains ATTXXXXX.htm attachment(s):

If you try to open these, by default they will launch in IE but not open anything or may not display anything meaningful. The reason for this is that the attachment is just a fragment of formatting code from the message body, i.e. html tags. You can see this by right-clicking in the page and selecting View Source:

The short answer as to why this happens is the way the message is being reformatted on the mail server that receives it. You often see this when Apple mail is sent to an Exchange\Outlook environment. It’s also common to encounter fragments of the message text as ATTXXXXX.txt attachments. Both these issues occur when text and attachment sections are combined with each other due to the way the sending application is encoding the message and the way in which the receiving mail server is decoding the message.

.dat attachments are a little different. These may be valid attachments and you can often tell by the size (KBs vs bytes). I have seen a rise in attachments like .doc or .pdf being converted to .dat with the rise in usage of mobile devices like iPhones/iPads for emailing. You can usually figure out the attachment type by opening it with a text editor and looking for the signature of the file type:

PK and .xml tell me this is a Word 2007/10 document:

Additional resources:

http://discussions.apple.com/message.jspa?messageID=11921402

The body of a message is shown incorrectly as an attachment if you try to use an application in an Exchange Server environment to send a message that includes attachments

Decoding Internet Attachments

ATT00001.htm attachments since 10.6.4