Published 2026-10-18 00:00:00 +0000 UTC · Updated 2026-11-05

What Is a .dat File and How to Open It

Unlike .pdf or .png, the .dat extension does not stand for one specific file format. Short for data, .dat is a generic extension used by hundreds of unrelated programs when saving attachments, configuration files, game assets, video streams or database tables.

Because any format can be named .dat, double-clicking it rarely works—and guessing an app at random can be frustrating. The right way to open a .dat file is to inspect its contents first.

Step 1: Check what the file actually is

Drop the .dat file into the free File Sniff file type checker. It reads the file’s magic bytes locally in your browser without uploading the file:

• A renamed PDF, Word document, ZIP archive or image: Email gateways, browser downloads and backup systems sometimes rename ordinary attachments to attachment.dat or document.dat. If the checker reports a standard format such as PDF document (%PDF-), PNG image or Word document (.docx), you can open a copy in the matching app. Read what a file extension mismatch means.
• An Outlook winmail.dat (TNEF) message: If the file arrived as an email attachment named winmail.dat (or ATT00001.dat) and starts with the bytes 78 9F 3E 22, it is a Microsoft Outlook Transport Neutral Encapsulation Format (TNEF) container.
• MPEG-1 / VCD video data: On older Video CDs, video tracks are stored as AVSEQ01.DAT using MPEG media streams.
• Plain text or JSON: Many software configuration and log files named .dat are readable UTF-8 text. Use the in-browser hex & text viewer to inspect the first bytes safely.

Step 2: Opening winmail.dat email attachments

When someone using Microsoft Outlook or Exchange sends an email formatted with Rich Text (RTF), Outlook bundles the formatting information and any attached files into a single TNEF package named winmail.dat. Apple Mail, Gmail, Thunderbird and iOS Mail do not decode TNEF packages natively, so recipients see a mysterious winmail.dat attachment instead of the PDF or spreadsheet the sender attached.

Use the free File Sniff winmail.dat attachment extractor to open winmail.dat right in your browser:

1. Drop winmail.dat onto the extractor.
2. The tool parses the TNEF header (78 9F 3E 22) and MAPI attachment records locally on your device.
3. Download the enclosed attachments with their original filenames (or save them all as a ZIP archive).

Step 3: When a .dat file is app-specific binary data

If the file came from inside an application’s program folder (such as Windows NTUSER.DAT, a game save folder, or an app cache) and shows no standard document or media signature, it is usually internal binary state meant only for that software. Do not modify or delete system .dat files.

To inspect the raw header bytes of an unfamiliar .dat file without opening it in an editor, use the file type checker or the in-browser hex viewer.