A file with no extension: how to tell what it is and open it
A file arrived with no extension or an odd name, and your computer does not know what to open it with. Usually it is a PDF, a document or a picture — you just need to find out which.
The extension may just be hidden
By default Windows and macOS hide the extensions of known types: report.pdf shows up as report. Turn extensions on and a lot becomes clear.
- Windows 11: File Explorer → View → Show → File name extensions.
- Windows 10: File Explorer → View tab → the File name extensions box.
- Mac: Finder → Settings → Advanced → Show all filename extensions.
Why a file has no extension
The extension gets lost when a file passes through some messengers and email programs, when a website sends a file without a name, and on hospital discs, where DICOM scans usually have no extension at all. The contents stay the same.
The type is written at the start of the file
Almost every format has a signature in its first bytes: a PDF starts with %PDF, a PNG picture with ‰PNG, and new Word, Excel and PowerPoint files with PK, because inside they are ZIP archives. The extension is only a hint in the name; the signature is what the file really is. That is why renaming .txt to .pdf does not turn text into a PDF.
Let the browser work it out
Drop the file on OsonKit or pick it with the button. OsonKit looks at the contents, not the name: it recognises PDFs, Word, Excel and PowerPoint documents, OpenDocument, RTF, pictures and DICOM scans, and opens the file in the right viewer straight away. A text file with no extension opens as plain text. The file is read in your browser and never uploaded.
If the format is not on that list, OsonKit says so and lists what it can open. Then look at the first characters: open the file in Notepad and check how it begins — for example, PK for ZIP archives or Rar! for RAR archives.