.DAT Data File
.dat

Data File

DAT is not a format — it is a filename convention shared by hundreds of unrelated applications. A winmail.dat is TNEF email (magic: 78 9F 3E 22), a VCD DAT is MPEG-1 video (magic: 00 00 01 BA), and NTUSER.DAT is a Windows registry hive (magic: regf). Run file(1) on any .dat to identify its true format.

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File structure
Header schema
Records structured data
Text1970
By FileDex
Not convertible

DAT file conversion is not applicable — DAT is a generic extension used by many applications with different internal formats. Identify the actual content type first, then handle it as that format.

Common questions

What is a DAT file?

A DAT file is a generic data file whose content depends on the application that created it. It could be a game save, an email attachment (winmail.dat), a video file (VCD), system cache data, or any other application-specific data. The .dat extension alone does not indicate the format.

How do I open a DAT file?

First, determine which application created the file. Try opening it in a text editor — if you see readable text, it is text-based. If the content looks like binary, open it in a hex editor (such as HxD on Windows) and compare the first few bytes against known file signatures to determine the actual format. For winmail.dat, use a TNEF decoding tool or website.

What is winmail.dat?

Winmail.dat is a TNEF-encoded email attachment created by Microsoft Outlook when sending messages in RTF format. Non-Outlook email clients cannot read TNEF, so attachments appear as a single winmail.dat file. Use online decoders like Winmaildat.com, or desktop tools like TNEF's Enough (macOS) to extract the original attachments.

Is it safe to open a DAT file?

DAT files from trusted sources (your own applications, games, email) are generally safe. DAT files from unknown sources should be treated with caution — the generic extension could hide malicious executables. Never rename a .dat file to .exe and run it. Scan unknown DAT files with antivirus software.

How do I identify what is inside a DAT file?

Run 'file unknown.dat' on Linux or macOS — it inspects magic bytes and reports the actual format (e.g., 'MPEG sequence', 'Transport Neutral Encapsulation Format', 'MS Windows registry file'). On Windows, open the file in HxD hex editor and compare the first 4-8 bytes against known signatures: 78 9F 3E 22 = winmail.dat (TNEF), 00 00 01 BA = VCD video (MPEG-PS), 'regf' = registry hive, 1F 8B = gzip-compressed data.

How do I play a VCD DAT video file?

VCD DAT files (typically named AVSEQ01.DAT in the MPEGAV folder) contain MPEG-1 Program Stream video at 352x288 resolution. Open them directly in VLC Media Player — no conversion needed. To re-encode as modern MP4, drop the DAT file into FileDex's converter and select MP4 as the output. The quality ceiling is limited by the original 1150 kbps source — conversion improves compatibility, not resolution.

What makes .DAT special

Since 1992
winmail.dat: Outlook's most hated attachment
Microsoft Outlook began embedding rich-text formatting in TNEF-encoded winmail.dat attachments in 1992. Three decades later, Gmail and Apple Mail users still receive unreadable winmail.dat files — the longest-running email compatibility problem in computing.
VCD era
Millions of family movies stored as AVSEQ01.DAT
Video CD stored MPEG-1 video in .dat files (352x288, ~1.1 Mbps). VCD dominated home video in Asia and the Middle East from 1993 to ~2005. Those DAT files are playable today — VLC and FFmpeg decode them natively.
System critical
NTUSER.DAT = your entire Windows profile
Every Windows user has an NTUSER.DAT file at C:\Users\<name>\NTUSER.DAT. It contains the HKEY_CURRENT_USER registry tree — desktop settings, app preferences, environment variables. Delete it and the profile is destroyed.
Not a format
Same extension, zero shared structure
A winmail.dat starts with 78 9F 3E 22. A VCD DAT starts with 00 00 01 BA. NTUSER.DAT starts with 'regf'. A game save DAT could be anything. The .dat extension carries exactly zero information about what is inside the file.

What is a DAT file?

DAT (Data) is a generic file extension used by many different applications to store data. The content and format of a DAT file depends entirely on the application that created it — it could contain plain text, binary data, configuration settings, or even media. Unlike formats such as PDF or MP3, .dat has no single universal specification; it is simply a convention for "data file."

Continue reading — full technical deep dive

Because any program can create a .dat file with any internal structure, opening one requires knowing which application produced it. The file's internal magic bytes (the first few bytes of the file) can often reveal the true format.

How to open DAT files

  • Identify the source application first — check which program created the file
  • Notepad / VS Code — Try opening as text; readable content will appear if it's text-based
  • HxD (Windows) — Hex editor for inspecting raw binary content and magic bytes
  • VLC Media Player — If the file is a VIDEO_TS.dat or VCD-format video
  • Microsoft Outlook — For winmail.dat attachments (TNEF-encoded email attachments)
  • File utility (Linux/macOS) — Run file filename.dat to auto-detect the format

Technical specifications

Property Value
Format Application-specific (no universal spec)
Content Text, binary, or mixed
Structure Varies by creator
Common Sources Outlook, games, system tools, databases
MIME type application/octet-stream (generic binary)
Magic bytes Varies — no fixed signature

Common use cases

  • Email attachments: winmail.dat files are created by Microsoft Outlook when sending rich-text emails to non-Outlook clients. They use the TNEF (Transport Neutral Encapsulation Format) encoding.
  • Game save data: Many video games store save states and progress in .dat files (e.g., save.dat, player.dat)
  • System files: Windows and Linux system utilities create .dat files for internal state (e.g., Prefetch data at C:\Windows\Prefetch\)
  • Application data: Software installers, databases, and backup tools use .dat for proprietary storage
  • Minecraft: Player and chunk data are stored in .dat files using the NBT (Named Binary Tag) format

Identifying the real format

If you have an unknown .dat file, these steps help identify its true format:

  1. Open in a hex editor: Look at the first 4–8 bytes. Common signatures include PK (ZIP), %PDF (PDF), FFD8FF (JPEG).
  2. Try renaming the extension: If you know the source application, rename to the expected format (e.g., .dat → .mp4)
  3. Use the file command: On Linux/macOS, file unknown.dat uses magic byte detection to identify file types
  4. Check file size: Very small files (< 1 KB) are often text-based config or log data

Security considerations

DAT files from unknown sources should be treated with caution. Because the extension reveals nothing about the content, malicious executables or scripts are sometimes distributed with a .dat extension to bypass file-type filters. Always scan .dat files from untrusted sources with antivirus software before opening them. Never rename a .dat file to .exe and run it unless you fully trust the source.

winmail.dat specifically

winmail.dat is the most commonly encountered .dat file by everyday users. It appears as an email attachment when Outlook sends messages in RTF format to recipients who use Gmail, Apple Mail, or other non-Outlook clients. Recipients can decode it using tools like TNEF's Enough (macOS), Winmaildat.com (online), or Letter Opener (iOS).

.DAT compared to alternatives

.DAT vs .BIN
Specificity of format
Both are generic extensions for application-specific data files. Neither implies any particular format or structure. DAT is slightly more common in Windows applications, BIN in firmware and disk images.
 Draw
.DAT (WINMAIL.DAT) vs .EML
Email attachment portability
EML files are standard RFC 5322 email messages readable by any mail client. Winmail.dat uses proprietary TNEF encoding that requires special tools or Outlook to decode.
 EML wins

Technical reference

MIME Type
application/octet-stream
Developer
Various
Year Introduced
1970
Open Standard
No

Binary Structure

DAT files have no universal binary structure. Content is entirely application-specific: a winmail.dat file uses Microsoft TNEF encoding with a signature of 78 9F 3E 22, a Minecraft .dat file uses NBT (Named Binary Tag) format with gzip compression, a VCD video .dat uses MPEG-PS framing. The only reliable way to determine a DAT file's structure is to inspect the first 4-16 bytes for known magic byte signatures, or to identify the source application. Some DAT files are plain text and have no binary framing at all.

1980Early computing establishes .dat as a generic convention for data files alongside .txt for text files1992Microsoft Outlook begins using winmail.dat (TNEF encoding) for rich-text email attachments1996Video CD (VCD) standard stores MPEG video in AVSEQ01.DAT files in the MPEGAV directory2009Minecraft adopts .dat extension for NBT-format world, player, and chunk data files2015Windows 10 continues using .dat files for Prefetch data, system caches, and registry hive backups
Identify DAT file type with file command other 
file unknown.dat

Uses magic byte detection to identify the true format of a .dat file. Works on Linux and macOS. Outputs a description like 'MPEG transport stream' or 'Microsoft Outlook TNEF'.

Extract winmail.dat attachments other 
tnef winmail.dat --directory=output/

Extracts original email attachments from a TNEF-encoded winmail.dat file into the specified output directory. Install tnef via package manager.

View hex dump of DAT file header other 
xxd -l 64 unknown.dat

Shows the first 64 bytes of a DAT file in hex and ASCII, allowing manual inspection of magic bytes to identify the format.

Convert VCD DAT video to MP4 ffmpeg 
ffmpeg -i AVSEQ01.DAT -c:v libx264 -crf 20 -c:a aac output.mp4

Transcodes a VCD-format DAT video file (MPEG-1 program stream) to H.264/AAC MP4 for modern playback.

MEDIUM

Attack Vectors

  • Malicious executables disguised with .dat extension to bypass file-type filters and email scanners
  • DAT files from untrusted sources may contain exploits targeting specific application parsers
  • Renaming .dat to .exe and executing it can run arbitrary code

Mitigation: FileDex does not open, execute, or parse these files. Reference page only.

Magic byte file type identification utility — the first tool for unknown .dat files
HxD tool
Free hex editor for Windows to inspect raw binary content of DAT files
macOS app for opening winmail.dat email attachments
GUI editor for Minecraft NBT .dat files with tree-view navigation