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How to open a mystery file with no extension

What can you do with a file attachment that has no file extension?

Sometimes you get a file, probably an email attachment, that has no file extension. Without something after the dot/period (.doc .docx .xlsx .pptx etc.) Windows doesn’t know what program to use.

This is most likely to happen with files sent from Mac users. However it happens there’s usually a solution.

Of course, you can just ask the sender or ask them to send again with a file extension but that’s not always possible or convenient.

Before tinkering with a ‘mystery file’ be sure that the sender is legitimate and the file doesn’t have any worm, virus or other nasty.

Here’s a recent example of an email with attachments …

Mystery File image from Opening a mystery file with no extension at

The first attachment is a Word document with .doc extension. The ‘old’ document formats like .doc aren’t very safe and best avoided, see Why Old Office documents should be banned

But the second file ‘Fight flight’ is a mystery with no extension – what to do?

Sometimes the email message will give you a hint or you’ll be able to make an educated guess. In this case we know that it’s most likely a PowerPoint slide or presentation.

To open the file you can double-click on it and choose the application to open it. That can be cumbersome.

A better method is to save the file to a temporary folder and rename the file to add a file extension. Then try opening the file with that extension. If it doesn’t work, try another extension.

Right-click on the mystery file, choose Save As … and save to a folder. Open that folder in Explorer and rename the file to give it an extension. Rename is on the right-click menu or press F2.

Assuming it is from PowerPoint; is it a .ppt file or the newer .pptx format? Maybe it’s one of the other PowerPoint formats like .pptm .pps etc?

In this case it’s probably .ppt (the old PowerPoint document extension) because the Word document in the email was .doc not the newer .docx. The larger 1MB size supports that guess since .pptx would probably be smaller.

So we changed the file name to a .ppt, double-clicked on the file and ‘lo! it opened in PowerPoint. Success.

If PowerPoint didn’t open the presentation, we’d have to try other possible extensions as mentioned above.

Advanced trick

If you’re really stuck, you can try peeking into the mystery file for clues. The older Office documents (.doc .xls .ppt) don’t reveal anything but the newer Office formats (.docx .xlsx .pptx etc) do have a little exposed information.

Open the mystery file in a text editor like Notepad. You’ll see a lot of ‘junk’ but amongst all that may be some plain text clues. Here’s an example from two parts of a simple Word .docx document.

Mystery File image from Opening a mystery file with no extension at

You can’t read the document (that’s compressed) but there are plain text mentions of ‘word‘ which is a big clue . Excel files will have ‘xl‘ and PowerPoint ‘ppt‘. All of these plain text clues are near the end of file.

Near the very top of any newer Office document is the text [Content_Types] .xml .

ZIP files are another common attachment type. A ZIP file in Notepad has the letters ‘PK‘ at the very start (the original author was the late Phil Katz) and the names of the compressed files are also in plain text.

Mystery File image from Opening a mystery file with no extension at

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