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I was just curious as to how Mac applications automatically associate file types with them. More specifically, if an application is installed using a drag-and-drop method, then how does it automatically associate certain file-types with it? For example, if I have an AVI file associated with QuickTime, then I download VLC, drag it to my applications folder, and finally open up the file, it will be opened with VLC. I understand how this would occur with a installer package, but just don't quite understand how dragging a file would create this association.

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It's based on Uniform Type Identifiers. A component of Mac OS X called LaunchServices maintains a mapping of UTIs to apps (and each app declares the UTIs it can support). Note that UTIs are hierarchical, so if one app claims to support ZIP files and another claims to support archives, they can both open ZIP files.

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In a nutshell, applications declare what types they can open in an Info.plist file stored within the .app package. The Finder reads this information when an application is opened or copied and reports it to OS X's Launch Services database.

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