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Sometimes you might download a ZIP file, then forget about it,
next time you try to open it up it prompts you for a password,
you've forgotten. Rather than trying to decompress every single ZIP
RAR, XAR, etc.) file that you download to see if it works without
the File browser (usually, Explorer or Finder) will
automatically analyze the file and show a little picture of a key
the ZIP archive file's graphical icon, proactively alerting the user.
nowadays analyze files in the background for search indexing
MSDN: Creating Icon Handlers
Microsoft's documentation on how to do this [krelnik, Dec 13 2008]
||Windows doesn't do any analysis beyond checking the file extension. This would be fairly easy to implement on other platforms, though.
||Spacecoyote is only partially correct. *By default* Windows only looks at the extension. But the Windows shell supports an extension called an 'icon handler' that can be used to build precisely what you describe here.
||Good plan. I think I may keep that link to MSDN for later - I can already think of a few ways I might use that...