marebay

Hi-- I'm not sure if this question belongs here but here goes--lately I've been getting emails from a friend with attachments with a ".dat" file extension. What is a ".dat" extension, are they safe to open and if so, how is it done? Not too long ago I had to get some major help from rridgely (thanks again) to get my computer cleaned up so I'm being uber-careful about what I open but I'm also curious what these are. Any help would be appreciated. Thanks