How To Open DAT Files (including Winmail.dat)

Often, we come across an unknown file type with a .DAT extension which our computer fails to open. These types of files are generally received through emails, as while re-routing an email, often the extension types of the attachments are saved to .DAT by default.

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What are DAT files?

DAT files are generic data files that store arbitrary data. This means these file types store some format of computer data, but it is difficult to say what type of data it contains without actually opening it. It can be a word document, an excel sheet or even a encoded video file. DAT file extension is surprisingly very common file format and is used by Microsoft Outlook to save attachments being sent through it. It is easy to read DAT files if we know the source, as in what program was used to create it. But if the source is unknown, these file types can be hard to decipher.

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How to open DAT files?

The easiest way to open a file with .DAT extension is to double click on it and let the computer decide which program is best suited. But that rarely works as the computer fails to decipher it and prompts the users to choose from a list of pre-installed programs. If the source of the file is known, then we can directly select the required one. For example, if we know that a particular DAT file is a Word document, we can choose Word, or notepad to open it. Similarly, if we know the particular file that we have is a video file, we can directly choose a video player to open it. But what if the actual file type or the source program is unknown?

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How to view DAT files with unknown source?

Trying to open a file with a .dat extension is more of a trial and error process. One has to try to open the particular file with all possible applications that could be an option to see which one works. Or it is commendable to ask the sender what particular file type it was originated in. Then the extension can be changed accordingly, or can be opened with the required application. If that is not an option, then there are other ways like all format file readers and file converters.


Winmail.dat is a compressed file format which is used for saving files in MS Outlook or MS Exchange Server. This is technically a Transport Neutral Encapsulation Format (TNEF). Outlook and Exchange automatically encode messages in TNEF and it is a protocol that is only supported by Microsoft. So users with other email client such as Gmail, Yahoo or Mac Mail will have trouble opening it directly.

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How To Open Winmail.dat and unknown DAT files?

There are a number of third party applications, and online services for users to view, edit and save DAT files. Some of them are listed below:

  • – This is a website that provides a free online service to users to read DAT files. Users can upload the raw .dat file to the site and it decompresses it and converts it into the actual readable file type in a matter of seconds. This converted file can be then saved, viewed and edited.
  • iOS Decoder – Solely made for the iPhones and the iPads, this app helps to decode DAT files while reading from iOS devices and makes it convenient for users to read them while going through their email.
  • Winmail Reader – This is a third party app that can be installed in Windows, which do not have Outlook Express. This DAT file reader serves the same purpose of decoding DAT files and making them readable.
  • A few converters can also be found for download or online use. These file conversion services, instead of opening the DAT file for reading, directly convert them to their original file format and let users save them that way for future use.
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There are also a number of other websites and applications that work as a DAT file viewer like But users should be cautious while downloading any third party applications from the web as spamming and viruses attacks have become very common these days.


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