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Files & Storage / File Explorer

Windows File Extension Reference

Quick answer

A file extension tells Windows which app usually opens a file, but it does not prove a file is safe. Show extensions in File Explorer, check whether the type is executable, and use the [file extension lookup](/tools/file-extension-lookup) or the matching guide before opening unknown downloads.

Practical PC editorial teamUpdated November 2, 202618 min readBeginner-friendlyAbout 10 to 30 minutes

Quick diagnosis

If several of these match what you see, you are in the right guide.

  • You do not know what a file extension means
  • Windows asks which app should open a file
  • A download has an extension that looks suspicious
  • You need to show extensions before opening attachments

Possible causes

  • Windows hides known extensions by default on many PCs
  • A file has no default app association
  • A file was renamed without changing its real format
  • An executable or script is disguised with a familiar-looking name

What you need

  • File Explorer
  • Settings → Apps → Default apps

What this problem usually means

The extension is the part after the last dot, such as .pdf or .exe. Windows uses it for default apps and icons. Attackers also rely on hidden extensions, which is why invoice.pdf.exe is not the same as invoice.pdf.

This reference groups extensions by behavior: documents, installers, scripts, archives, images, media, disk images, web/data files, logs, temporary files, and shortcuts.

Before you start

  • In File Explorer, choose View → Show → File name extensions.
  • Do not open unexpected executable or script files from email, chats, or ads.
  • If a file will not open, do not rename the extension as the first fix. Find the app or get a clean copy.

Quick fixes

1. Show file extensions

  1. Open File Explorer.
  2. Choose View → Show → File name extensions.
  3. Look at the full filename, including the final extension after the last dot.
  4. If the extension looks unfamiliar, look it up before opening the file.

2. Use the extension table safely

  • .exe, .msi, .bat, .cmd, and .ps1 can run instructions on the PC.
  • .dll, .sys, .tmp, and .lnk are often support/system files; deleting them can break apps or shortcuts.
  • .zip, .7z, .rar, .iso, and .img are containers. The files inside still matter.
  • .pdf, Office files, images, and media are normally documents, but links/macros/download origin still matter.

Step-by-step troubleshooting

Common executable and system extensions

  • .exe: Windows program. Run only from trusted publishers or official sites.
  • .msi: Windows installer package. Use the vendor's real installer and avoid bundled download sites.
  • .bat / .cmd / .ps1: scripts that can change system settings. Do not run scripts from comments or forums unless you understand them.
  • .dll: library used by programs. Do not download single DLL files from random sites; repair or reinstall the app instead.
  • .sys: driver or system file. Treat unexpected .sys downloads as suspicious.

Documents, PDFs, and Office files

  • .pdf: document for viewing/printing. If it will not open, use PDF files won't open.
  • .docx / .xlsx / .pptx: Microsoft Office document formats. Be careful with files that ask to enable macros or install add-ins.
  • .txt / .csv: plain text or comma-separated data. They are usually safe to view, but content can still contain malicious links.
  • .log: text record produced by apps or Windows. Useful for support; usually not something you run.

Archives and disk images

  • .zip / .7z / .rar: compressed archives. Scan or inspect contents before opening executables inside.
  • .iso / .img: disk images. Windows can mount many of them. Be cautious if an image contains installers from an unknown source.

Images, media, web, and data

  • .jpg / .jpeg / .png / .webp: image files used by cameras, screenshots, and websites.
  • .mp4 / .mp3: video and audio files. Use a trusted media player and be careful with codec prompts from unknown sites.
  • .json / .xml: structured data used by apps and websites. Open in a text editor if you only need to inspect it.
  • .html / .htm: webpage file. It may open in a browser and can contain links or scripts.
  • .tmp: temporary file. Do not delete from program folders unless a cleanup guide says it is safe.
  • .lnk: shortcut. A shortcut can point to a program or script, so inspect unexpected shortcuts from downloads.

Should you open the file?

Choose an answer. The next question appears below, no account or download required.

Did you expect this exact file from this sender or website?
Is it executable or a script?

Run only if it came from the official publisher and you intended to install or run it.

Open with the expected app, but stay cautious with macros, links, and archives containing programs.

Do not open it yet. Check the extension, source, and Windows Security scan first.

How to know the problem is fixed

  1. File extensions are visible in File Explorer.
  2. Windows opens the file with the app you intentionally chose.
  3. You understand whether the file is executable, a document, an archive, or a support/system file.

If the problem continues

If Windows still cannot open the file, get a fresh copy from the sender or official source. Use the file extension lookup for common types, and avoid renaming files unless you know the original extension was hidden or changed by mistake.

When this may be hardware failure

A single unknown extension is not a hardware problem. If many file types become corrupt, vanish, or fail to copy, check storage health with the drive guides.

Common mistakes to avoid

  • Assuming an extension proves safety.
  • Running invoice.pdf.exe because Windows hid extensions before.
  • Downloading single DLL files instead of repairing the program.
  • Renaming .zip to .pdf to force it open.

Frequently asked questions

Can a PDF or Office document be dangerous?

It can contain malicious links, macros, or exploit content. The extension alone does not prove safety. Trust source, context, and Windows Security warnings.

Why did Windows ask me which app to use?

There may be no default app for that extension, or the file type is uncommon. Choose a trusted app or look up the extension before installing anything new.

Written by Practical PC editorial team. We write practical Windows and PC troubleshooting guides for home users, students, and office workers. Guides are researched from official documentation and widely observed support patterns, then reviewed for safety and plain language. We do not invent lab tests, certifications, or first-hand repair experience we do not have.