A File’s Name Is Not Its Contents
An extension such as `.txt` or `.png` is a clue attached to a filename, not a transformation of the bytes stored inside the file. Renaming `notes.txt` to `notes.png` changes that clue; the words in the file remain words encoded as text. No image pixels, color values, or image-compression structure appear just because the name now ends in `.png`.
That distinction matters because a computer uses file information in more than one way. The operating system may use the extension to choose which program to launch when you open a file. The program then has to interpret the actual contents. An image viewer expecting image data may reject a text file, show an error, or—in some cases—display misleading garbage if it tries to read the bytes as pixels.
Conversion Changes the Data
A real conversion requires software to read the original format and write a new one. For example, an image editor can decode an image into its visual information, then encode that information in another image format. Simply changing the suffix skips that process.
Extensions are still useful: they help people and software identify likely file types. But they are labels, not guarantees. When a file will not open, changing its name may affect which app tries to handle it; it cannot repair or replace the data that app needs to find inside.
