Manually changing a file extension modifies the letters after the last dot in a filename. File extensions act like labels telling your operating system which program to use to open the file. Changing it doesn't alter the file's actual content or format; it only changes how your computer tries to interpret it. If the new extension doesn't match the file's true internal structure, the associated application will usually fail to open it correctly, displaying an error message or showing corrupted data. Renaming a .txt file to .jpg doesn't make it a valid image.

For instance, changing a text file notes.txt to notes.jpg causes an image viewer to fail when trying to display it. Conversely, strategically renaming a .docx file (which is a packaged format) to .zip allows accessing its internal XML files and resources using archive software, since .docx files are technically ZIP archives. This renaming is often done manually via a computer's file explorer.

While sometimes useful for forensic exploration or format tricks, manually changing extensions carries significant risks. An incompatible application might appear to open the file but display gibberish or corrupted content. Crucially, some programs might overwrite the file incorrectly when saving, potentially causing irreversible data loss. Always make a backup copy before experimenting. Modern operating systems increasingly rely on content detection alongside extensions, but file extensions remain fundamental for initial file association.