File formats that open directly in the browser

When a recipient opens a link, the content should ideally appear right away, with no download and no extra software. That depends on the file format, because the recipient's browser is what displays it.

Images

JPG, PNG, GIF and WebP open in every current browser. AVIF is also supported by recent versions of Chrome, Firefox, Safari and Edge.

Photos taken on an iPhone are often saved as HEIC, a format most browsers do not display. Convert to JPG before sending or set the camera to its most compatible mode. RAW, PSD and TIFF files also need to be exported.

Videos

The safest combination is MP4 with H.264 video and AAC audio: it plays on computers, phones and TVs. WebM also works in current browsers.

The extension does not tell the whole story. A MOV or MP4 file recorded in HEVC (H.265), the default on many phones, may play on one device and show a black screen on another. MKV, AVI and WMV usually do not play. When in doubt, convert to H.264 MP4 with a free tool such as HandBrake.

Audio

MP3, M4A (AAC) and WAV play almost everywhere. FLAC and OGG work in current browsers but may fail in older versions of Safari. For voice and general music, MP3 at 192 kbps is a good balance between quality and size.

Documents and text

PDF is the only document format every browser displays with its built-in reader. Word, Excel and PowerPoint files only open with the program installed, so export to PDF when the recipient just needs to read.

Plain text files such as TXT, CSV and JSON are shown as text. On fstmp the preview shows the first 256 KB, and HTML and SVG files are displayed as source code, without being executed, for safety.

If the preview does not appear

fstmp delivers the file exactly as it was uploaded, with no conversion. When the browser does not recognize the format, the page shows a notice and the download button remains available, so the recipient can always get the original.

To avoid surprises, open the link on another device before sharing it, preferably a phone, which is where most links are opened.