Which app opens which file? A plain-English file type reference

When a file won’t open, the cause is nearly always one of two things: the device has no app that understands that format, or the file has the wrong extension. This reference lists the most common formats people ask about, what they are, and what opens them on Windows, Mac, iPhone and Android. Where a format is awkward, there is a free converter next to it.

The file type table

App names reflect what is commonly available at the time of writing. Support for newer image formats has been spreading fast, so an up-to-date operating system or browser often opens more than the table suggests.

How to tell what a file really is

The extension is just part of the name, and it can be wrong or missing. A file called photo.jpg may really be a HEIC or WebP image that someone renamed, and a download with no extension at all is common from email and messaging apps.

• Check the extension is visible. On Windows, open File Explorer, choose View, then tick “File name extensions”. On a Mac, open Finder Settings, Advanced, and tick “Show all filename extensions”.
• Look at the size and source. A “JPG” that opens nowhere but is huge or came from an iPhone is probably HEIC.
• Try a browser. Drag the file onto a Chrome, Edge or Safari window. Browsers open PDFs, most images, text, SVG and JSON without any extra software.
• Don’t just rename it to force it. Renaming .heic to .jpg only changes the label, not the contents, so many apps will still refuse it. Convert it instead.

Common problems and the quickest fix

“I can’t open a PDF to edit it”

PDFs open everywhere but are built to be read, not edited. To change the text, convert it to Word with PDF to Word. Scanned PDFs are pictures of pages, and need text recognition first.

“My iPhone photos won’t open on a Windows PC or upload to a form”

iPhones can save photos as HEIC. Convert them with HEIC to JPG, or set the camera to Most Compatible under Settings, Camera, Formats.

“An image downloaded as WebP and my editor rejects it”

Websites often serve WebP or AVIF images. Use WebP to JPG, WebP to PNG or AVIF to JPG to get a format any app accepts. If you need transparency, choose PNG. See JPG vs PNG for which to pick.

“A DOCX looks wrong or won’t open”

Pages, Google Docs and LibreOffice can open DOCX but may shift fonts and spacing. To share a document that looks the same for everyone, send it as a PDF with Word to PDF.

“A CSV opens as one messy column”

Some regions use semicolons rather than commas, so Excel can put everything in column A. Import the file with Data, From Text/CSV and choose the right delimiter, or convert it to structured data with CSV to JSON.

Why a browser-based converter is the safe choice

Many online converters upload your file to a server. For a holiday photo that may not matter, but contracts, ID scans and invoices are another matter. Altto does the conversion inside your browser tab, so the file is never sent anywhere. See how to convert a PDF to Word without uploading it for how that works.