A DAT file is simply a file that contains data. That data could be plain text, or it could be the binary encoding of an image, video, or other file. For example, some email tools provide image attachments as DAT files.
Since you don't know what type of data you're dealing with, one way to handle a DAT file is to first open it in a text editor (like Notepad) and look for encoding clues. Once you know the encoding format, you can change the file extension and allow your system's default handler to open the file.
Determine the Data Type
Launch Notepad, select File and then Open.
Within the Open dialog, set the filter to All Files and then open the DAT file.
If the file is plain text, then you're done: it will be human-readable. Otherwise, look for clues in the binary data about the file's encoding or compression type, or the tool that created it. If you see text like "GIF", "PNG" or "JFIF," then you know it's an image file. If you see text like "mp4" or "avi," then the file is a movie. For example, we know that the following is a GIF image file because it contains the text "GIF".
Windows supports a wide range of other common image and video formats.
Change the File Extension
Once you know the data type, rename the file with the correct extension so your system will know how to open it. If you weren't able to deduce the type, try renaming the file to a common image or video extension anyway to see if it will open.
From the Windows Control Panel, select Appearance and Personalization and then Folder Options. Under Folder Options, select Show hidden files and folders.
Under Advanced Settings, deselect the option Hide extensions for known file types and click OK.
Finally, right click on the file and select Rename. Change the file extension from DAT to the correct type for the encoding format, click Yes to accept the change, and double click the file to open it.
You may also run into a DAT file as a winmail.dat attachment in a received email. This can happen if the sender uses a Microsoft email client (e.g. Microsoft Outlook, Microsoft Exchange Client) that uses Transport Neutral Encapsulation Format (TNEF) or Microsoft Outlook Rich Text Formatting (RTF), but your email tool can't decode it.
To open this type of DAT file, upload it to an on-line reader like WindmailDat, or use one of several windmail.dat reader apps available in the iTunes Store or the Google Play Store.