What is a file extension?
The extension is the part after the last dot in the file name, such as .pdf in "Invoice.pdf". It tells the operating system which program should open the file. Changing only the extension does not change the content – renaming a HEIC file does not turn it into a JPEG.
Windows often hides extensions. You can show them in File Explorer under View → Show → File name extensions.
Images: JPG, PNG, HEIC, WEBP and more
| Extension | What is it? | Open with | Preview in SafeDoc |
|---|---|---|---|
| .jpg | Photo in JPEG format, the most common image format. Lossy compression. | Any browser and any image viewer | Yes, as an image |
| .png | Lossless image format with transparency. Good for graphics and screenshots. | Any browser and any image viewer | Yes, as an image |
| .heic | Photo format of iPhone and iPad (HEIF). Smaller than JPEG at the same quality. | Apple devices; on Windows the Photos app with the HEIF extension | No – download and open |
| .webp | Modern image format for the web, usually much smaller than JPEG. | All current browsers, most image programs | Yes, as an image |
| .avif | Very new image format with strong compression. | Current browsers; older image programs often not | Yes, as an image |
| .gif | Old image format with at most 256 colours, often used for small animations. | Any browser | Yes, as an image |
| .svg | Vector graphic – stays sharp at any size. Technically a text document that can also contain program code. | Browsers, Inkscape, Illustrator | No – download and open |
Photos too large to send? Resize images for free in your browser – and with remove location from photos an image no longer reveals where it was taken.
Documents: PDF, DOCX, ODT, XLSX, CSV
| Extension | What is it? | Open with | Preview in SafeDoc |
|---|---|---|---|
| Document that looks the same on every device. Standard for invoices and forms. | Any browser, Adobe Acrobat Reader, Preview (Mac) | Yes, as a PDF | |
| .docx | Text document from Microsoft Word (since 2007). | Word, LibreOffice Writer, Pages, Google Docs | No – download and open |
| .doc | Older Word format (up to 2003). | Word, LibreOffice Writer | No – download and open |
| .odt | Open text format (OpenDocument), standard in LibreOffice. | LibreOffice Writer, Word | No – download and open |
| .xlsx | Spreadsheet from Microsoft Excel. | Excel, LibreOffice Calc, Numbers | No – download and open |
| .pptx | Presentation from Microsoft PowerPoint. | PowerPoint, LibreOffice Impress, Keynote | No – download and open |
| .txt | Plain text without formatting. | Any text editor | Yes, as text (up to 512 KB) |
| .csv | Table as plain text, values separated by commas or semicolons. Typical for exports from banking and shops. | Excel, LibreOffice Calc, any text editor | Yes, as text (up to 512 KB) |
| .md | Markdown: text with simple markup such as # for headings. | Any text editor, Obsidian, Typora | Yes, as text (up to 512 KB) |
| .epub | E-book format that adapts the text to the screen size. | Apple Books, Calibre, most e-readers | No – download and open |
Archives: ZIP, 7Z, RAR
| Extension | What is it? | Open with | Preview in SafeDoc |
|---|---|---|---|
| .zip | Compressed archive with several files and folders. | Built into Windows and macOS, 7-Zip | No – download and open |
| .7z | 7-Zip archive, often smaller than ZIP. | 7-Zip, NanaZip, Keka (Mac) | No – download and open |
| .rar | Archive format of WinRAR. | WinRAR, 7-Zip, current Windows versions | No – download and open |
| .gz | Compressed single file, often .tar.gz for whole folders (Linux). | 7-Zip, built into macOS and Linux | No – download and open |
An archive from an unknown source can contain malware. After downloading, check the checksum if the provider publishes one.
Video and audio: MP4, MOV, MKV, MP3
| Extension | What is it? | Open with | Preview in SafeDoc |
|---|---|---|---|
| .mp4 | The most common video format, also used by phones. | Any browser, VLC, any video player | Yes, play video |
| .mov | Apple video format (QuickTime), standard for iPhone videos. | QuickTime, VLC, Photos app | No – download and open |
| .mkv | Matroska: video container that can hold several audio tracks and subtitles. | VLC, mpv, Films & TV (Windows) | No – download and open |
| .webm | Open video format for the web. | Any current browser, VLC | Yes, play video |
| .mp3 | The most common music and audio format. | Any browser and any audio player | Yes, play |
| .m4a | Audio format (AAC), e.g. iPhone voice memos. | iTunes/Music, VLC, most players | No – download and open |
| .flac | Lossless audio format in full CD quality. | VLC, foobar2000, most players | No – download and open |
Which files does SafeDoc show in the preview?
You can store files of any kind in SafeDoc. You can view directly in the browser the formats that browsers themselves display safely: common images (JPEG, PNG, GIF, WebP, AVIF), PDF, videos in MP4 and WebM, audio in MP3, OGG and WAV, and text files.
Office documents, HEIC photos, MOV and MKV videos and archives are downloaded and opened with the right program. We deliberately do not show SVG graphics directly because they can contain program code.