Apple Mail detects ".dat" files as "embedded text" or metadata for attachments. When you open the file the first 3 lines should look something like this:
company name : First Last
address : 123 Main Street
zipcode : 12345
Populated with the information you provided during registration. If the first 3 lines of your license does not look similar, you will need to download the file using WebMail or something other than Apple Mail (eg - Thunderbird).