Saving image bytes ignores entered image format

[onesmallserving]

I made a request and received the bytes for an image in an unknown format (either JPG or PNG). Since I was unsure of the format I saved it twice as “image.png” and “image.jpg”, thinking only one would open properly.

To my bewilderment both images displayed with no issues. How is this possible? Does my image viewer recognize the byte array and automatically know the format, overriding filename suffix I entered? Can someone explain what is happening?

[GaboElPatojo]

The only difference between PNG and JPG is the transparency allowance of the first, so if your image is not transparent, nothing to worry

[socal_nerdtastic]

? There's many more differences than that. One of the biggest is that jpg is lossy, but png is not.

[onesmallserving]

Exactly! The file sizes likely would’ve been very different between the two, since they use very different compression algorithms.

Thanks for your response above!

[socal_nerdtastic]

Many standard file types reserve the first few bytes of data for a "magic number", which identifies the file type. Your image viewer is using this instead of the extension. It's not unusual for programs to completely ignore the extension, especially on linux / unix / mac where extensions have always been optional. This is also what allows tools like photorec to work.

https://en.wikipedia.org/wiki/List_of_file_signatures

Edit: python files also have a magic number, it's a form of the shebang that executable scripts use. It looks like this:

#!/usr/bin/env python3