Archive & Compressed Files
Files that contain other files packed together.
| Extension | What It Is | How to Open |
|---|---|---|
| .zip | The most common compressed archive format | Full guide — Windows built-in, 7-Zip, or WinZip |
| .rar | RAR archive — similar to ZIP, slightly better compression | Full guide — 7-Zip, WinRAR, or WinZip |
| .7z | 7-Zip archive — high compression, strong encryption | Full guide — 7-Zip (free), The Unarchiver (Mac) |
| .tar.gz / .tgz | Compressed tarball — common in Linux/Unix | Full guide — 7-Zip (Windows), double-click (Mac) |
| .tar | Uncompressed tarball — a bundle of files | 7-Zip (Windows), double-click (Mac) |
| .gz / .gzip | Gzip compressed single file | 7-Zip (Windows), double-click (Mac) |
| .bz2 | Bzip2 compressed file — better compression than gzip | 7-Zip (Windows), double-click (Mac) |
| .xz | XZ compressed file — high ratio compression | 7-Zip (Windows), The Unarchiver (Mac) |
| .cab | Windows Cabinet file — used by Windows installers | Windows built-in, or 7-Zip |
Disk Images
Virtual copies of CDs, DVDs, or drives.
| Extension | What It Is | How to Open |
|---|---|---|
| .iso | Disc image (CD/DVD/Blu-ray copy) | Full guide — Double-click on Win 10/11 or Mac |
| .dmg | Apple Disk Image — Mac software distribution format | Full guide — Double-click on Mac; 7-Zip on Windows |
| .img | Raw disk image | 7-Zip, or mount with Disk Utility (Mac) |
| .vhd / .vhdx | Virtual hard disk (used by Windows Hyper-V) | Windows Disk Management → "Attach VHD" |
Email & Communication Files
Files exported from email clients.
| Extension | What It Is | How to Open |
|---|---|---|
| .pst | Outlook data file — contains emails, contacts, calendar | Microsoft Outlook (import via File → Open & Export) |
| .ost | Offline Outlook data file | Microsoft Outlook (opens automatically) |
| .msg | Single Outlook email message | Microsoft Outlook, or free MSG Viewer tools online |
| .eml | Standard email message file | Most email clients (Outlook, Thunderbird, Apple Mail) |
| .mbox | Mailbox file — collection of emails | Thunderbird, Apple Mail, or Google Takeout import |
Data & Database Files
Structured data and common "mystery" files.
| Extension | What It Is | How to Open |
|---|---|---|
| .dat | Generic data file — could be anything | Depends on context. Try opening with Notepad/TextEdit first. Ask the sender what program created it. |
| .csv | Comma-separated values — spreadsheet data | Excel, Google Sheets, or any text editor |
| .json | JavaScript Object Notation — structured data | Any text editor (Notepad, VS Code, TextEdit) |
| .xml | Extensible Markup Language — structured data | Web browser or any text editor |
| .sql | Database query/export file | Any text editor, or import into a database program |
| .db / .sqlite | Database file | DB Browser for SQLite (free) |
Other Common File Types
Files people frequently ask about.
| Extension | What It Is | How to Open |
|---|---|---|
| .exe | Windows executable (program/installer) | Double-click to run on Windows. Does NOT work on Mac. |
| .apk | Android app package | Transfer to Android device to install. Not for PC/Mac. |
| .torrent | BitTorrent link file (not the actual content) | A BitTorrent client like qBittorrent or Transmission |
| .log | Log/text file | Any text editor (Notepad, TextEdit) |
| .bak | Backup file | Depends on the software that created it |
| .tmp | Temporary file — usually safe to delete | Generally not meant to be opened |
| .bin / .cue | CD/DVD disc image (older format) | 7-Zip to extract, or mount with virtual drive software |