Decode Any File.
Understand Everything.

A file extension is the suffix at the end of a filename, after the final dot, for example .pdf, .jpg, or .exe. It is a label that tells your operating system what type of data the file contains and which application should open it. The extension does not change the actual file data, it is purely informational. Your OS reads it and looks up the associated application in a registry. Without this system, your computer would need to scan every file's raw bytes to determine how to handle it, which would be extremely slow.

No, and this is the most important thing to understand about file extensions. Renaming a file extension changes only the label on the outside; the internal binary data remains completely unchanged. If you rename photo.png to photo.jpg, the file still contains PNG data internally. Programs expecting JPEG data will either fail to open it or display corrupted output. For genuine format conversion, use a dedicated tool: HandBrake for video, FFmpeg for audio, LibreOffice for documents, GIMP for images, or one of the online converters listed on each extension page in our database.

Start by searching the extension in the Filezeno search bar at the top of this page. We document 10,000+ extensions with full software compatibility information for every platform. If you find a match, the extension page will list all compatible software and include links to free downloads. If the extension is not in our database, try searching "[extension] file format" in a search engine. As a last resort, many archive managers (like 7-Zip) can attempt to open unknown formats by reading the file's internal structure.

It depends on the operating system. On Windows, extensions are not case sensitive, .JPG, .jpg, and .Jpg are all treated identically. On macOS, extensions are also generally case-insensitive for application associations, though macOS preserves the case in the actual filename. On Linux, extensions are fully case-sensitive, .jpg and .JPG are different files and are treated as different types. For maximum cross-platform compatibility, always use lowercase extensions. This is particularly important for web files, which are served by Linux-based servers where case matters.

The extensions most commonly associated with malware and social engineering attacks are: .exe (Windows executable program), .bat (Windows batch script), .vbs (Visual Basic Script), .scr (screensaver, actually an executable), .ps1 (PowerShell script), .jar (Java executable), .cmd (Windows command script), .msi (Windows installer), .pif (program information file, can execute code), and .hta (HTML application). Never open files with these extensions if they arrived via email from an unknown sender, were downloaded from an unfamiliar website, or appeared unexpectedly on your system. Attackers frequently bundle these inside .zip files to bypass email filters.

Microsoft introduced this behavior to make the interface appear cleaner and less technical for mainstream users. The reasoning was that most users do not need to see .exe, .txt, or .jpg suffixes on every file. However, this creates a well-documented security vulnerability: it makes it trivially easy for attackers to disguise malicious files. A file named document.pdf.exe appears to Windows users as document.pdf. To disable this setting and see all extensions: open File Explorer, click the View tab, and check the "File name extensions" checkbox. On Windows 10 and earlier: go to Folder Options → View tab → uncheck "Hide extensions for known file types." This change takes effect immediately and applies globally.

A container is the file format wrapper that holds multiple data streams, a video stream, one or more audio streams, subtitles, chapter markers. The extension identifies the container: .mp4, .mkv, .avi. A codec is the algorithm used to compress and decompress each individual stream, H.264, H.265, AV1 for video; AAC, MP3, AC3 for audio. The container and codec are independent choices. This is why two .mp4 files can behave completely differently: one might use H.264 video with AAC audio (plays everywhere), while another might use H.265 video with AC3 audio (requires specific hardware or software support). When exporting video for broad sharing, choose MP4 container with H.264 video and AAC audio for maximum compatibility.

There is absolutely no technical difference. Both .jpg and .jpeg refer to the same file format: JPEG (Joint Photographic Experts Group). The shorter .jpg variant became the standard on older Windows and DOS systems, which required file extensions to be three characters or fewer. Modern operating systems handle both identically. You can rename a .jpeg file to .jpg (or vice versa) without any effect whatsoever on the actual image data. Most software accepts both extensions and treats them as the same format.

For photographs and complex images: .webp is the modern answer. It achieves 25–35% smaller file sizes than JPEG at equivalent visual quality, supports both lossy and lossless modes, and has full browser support across Chrome, Firefox, Safari, and Edge. For graphics, icons, and logos with transparency: .svg (vector) is ideal when you have it, otherwise .png. For animated content: .webp animations are more efficient than .gif and should be preferred for new work. Avoid .bmp and uncompressed .tiff on websites, these load extremely slowly due to their large file sizes.

Lossy compression permanently removes some data from the file to achieve smaller sizes. The removed data is gone forever, it cannot be recovered. Every time a lossy file is re-edited and re-saved, quality degrades further. JPEG (.jpg) for images and MP3 (.mp3) for audio are lossy. At high quality settings, the degradation is typically imperceptible to most people, but it exists. Lossless compression reduces file size by finding and eliminating redundancy in the data, without discarding anything. The original data can be perfectly recovered by decompressing. PNG (.png) for images and FLAC (.flac) for audio are lossless. For archiving anything that matters, original photographs, master recordings, important documents, always use lossless formats.

For documents that others should read but not edit: .pdf is the definitive answer. A PDF renders identically on every device and operating system, preserves all formatting exactly, and cannot be accidentally modified without the right tools. For editable documents that will be worked on collaboratively: .docx offers the broadest compatibility, Microsoft Word, Google Docs, LibreOffice Writer, and most other word processors open it. For plain text notes or simple data without formatting: .txt is universally readable on every device ever made. Avoid proprietary formats like .pages (Apple only) or .wps (WPS Office only) when sharing with people whose software you do not know.

There is no definitive answer, but estimates from researchers range from 10,000 to over 50,000 active extensions when including proprietary, niche, regional, and legacy formats. New extensions are created continuously as new software is developed, and old ones become obsolete as applications are discontinued. Filezeno documents 10,000+ of the most commonly encountered extensions, the ones most people will realistically come across. Our database is updated continuously as new formats emerge and existing ones change.

Yes, and this is a genuine source of confusion. File extensions are not guaranteed to be unique. For example: .ts can be a TypeScript source file or an MPEG Transport Stream video. .log can be a plain text log file or a binary database transaction log. .raw can be a camera raw image or raw audio data or raw binary data. .db can be a SQLite database, a Thumbs.db thumbnail cache, or various other database formats. When an extension is ambiguous, the actual file contents, specifically the first few bytes known as the "magic number" or file signature, are the definitive way to identify the true format. Filezeno documents all known meanings for each extension.

.mp4 with H.264 video encoding and AAC audio encoding is the universal standard for sharing video. It plays natively on every smartphone (iOS and Android), every computer (Windows, Mac, Linux), every smart TV, and every streaming platform without additional software or configuration. For archiving high-quality video with multiple audio tracks or subtitle streams, .mkv is the better container choice. For web-embedded video in HTML5 without licensing concerns, .webm is the open-source standard. For new projects, avoid .avi and .wmv, they have poor compatibility with modern mobile devices and streaming systems.

Three free tools cover the vast majority of unknown files. VLC Media Player (available at videolan.org) plays virtually every audio and video format ever created, if VLC cannot open a media file, it is genuinely exotic. 7-Zip (available at 7-zip.org) opens ZIP, RAR, 7z, TAR, GZ, BZ2, and 30+ other archive formats. LibreOffice (available at libreoffice.org) opens Microsoft Word (.docx), Excel (.xlsx), PowerPoint (.pptx), and most other document formats at no cost. Together, these three applications handle the overwhelming majority of unknown files encountered in everyday computing.

A file extension is a suffix added to the end of a filename that indicates its type and format. For example, .docx indicates a Microsoft Word document.

Check the file extension and use the appropriate software indicated on our site. We provide compatibility information for Windows, Mac, and Linux.

On Windows, file extensions are not case sensitive, unlike macOS and Linux. Always check the exact extension.

Try other software suggested on the file extension page. Some files may require specific software or codecs.