How to Change a File Extension in Windows, Mac & Online

A file extension is the suffix at the end of a filename (such as .txt, .jpg, .pdf, or .json) preceded by a period. It informs your operating system which application should open the file by default.

Frequently, files downloaded from web apps, messaging tools, or cameras lack an extension or have an incorrect format label. Knowing how to change file extension correctly restores file associations without corrupting your data.

By default, Windows hides known file extensions to prevent accidental renames. To change a file type in Windows, you must first unhide extensions:

1. Open File Explorer (press Win + E).
2. Click the View tab at the top menu.
3. Hover over Show and check File name extensions.
4. Locate the file you wish to rename, right-click it, and select Rename (or press F2).
5. Change the letters after the dot (e.g., from sample.txt to sample.md) and press Enter.
6. When Windows displays the prompt 'If you change a file name extension, the file might become unusable', click Yes.

Changing file suffixes on a Mac is straightforward within Finder:

1. Open Finder, click Finder in the top menu bar, and choose Settings (or Preferences).
2. Select the Advanced tab and check the box labeled Show all filename extensions.
3. Click the file you want to edit and press Return (Enter) to rename it.
4. Highlight the existing extension after the period and type the new extension.
5. Press Return and confirm the change by clicking Use .[new extension] in the prompt.

It is crucial to understand that changing a filename extension is purely an operating system labeling change:

• Safe renames: Changing text-based formats (e.g., .txt to .md, .csv to .txt, or .html to .htm) is completely safe because the underlying plain text structure is identical.
• Invalid renames: Renaming photo.jpg to document.pdf or audio.mp3 does NOT convert the underlying binary data. To convert media formats, you must use a dedicated converter tool.

If you have dozens of files that need extension renaming or if you are on a restricted mobile device, doing this manually file-by-file is exhausting.

You can use ToolNova's free File Extension Changer to drag and drop single or multiple files simultaneously, choose your desired target suffix, and download all renamed files in a single zip archive. The entire process runs 100% locally in your web browser, ensuring zero files are uploaded to any server.