File extension settings under Windows and Mac
By default, Windows and Mac both hide file extensions, such as the '.png' that you need to put at the end of all your Portable Network Graphics image filenames, the '.zip' that you need to put at the end of all your compressed files, the '.py' that should be at the end of your Python source code files, etc.
Windows and Mac also both tend to often launch the wrong application by default when you double-click a file.
These defaults often causes problems for designers and developers, since we usually want to control the file extensions explicitly, not have them hidden as if we weren't smart enough to know what they are. We also usually want to control explicitly which application is launched when you double-click a file, and not leave it up to the operating system to decide.
Here are some steps you can take to make your computer deal more transparently with file extensions and default applications:
Change operating system settings to show all file extensions
Find instructions on how to show file extensions for whichever operating system you prefer.
When in doubt, open files from within the application you want to use, not by double-clicking the files
Your computer probably will at some point open the wrong application when you double-click on a file you want to edit.
The easiest, most reliable way around this problem is to always open your files by first running the application you want to use, and then going to File->Open and selecting the file, rather than double-clicking the files in Windows Explorer or Mac Finder. But if you really want to be able to double-click to run them, change the settings as outlined in the next section.
Change operating system settings to launch the appropriate application when you double-click a file
Use a web browser and a search engine (preferably one of those that do not track and profit from your "private" online behavior and data) to find instructions on how to change file associations in whichever operating system you prefer.