People use Finder to access the macOS file system.

Make sure your app has a filename extension of .app. The Finder uses this extension to identify and interact with apps. Note that file extensions may be hidden from view depending on the user’s Finder preferences.

Make sure your app’s files include appropriate filename extensions. Extensions identify files, affect how files are handled by the system, and help ensure platform interoperability. In addition to an extension, you can set a file type and optionally a creator type for a file, although this is not strictly necessary. Note that regardless of how your files are defined, the user can manually associate a specific type of file with a specific app.

Provide a Quick Look generator if your app creates custom or uncommon types of documents. A Quick Look generator produces previews of specific document types, which the Finder displays in Cover Flow views and Quick Look preview windows. If your app produces documents in formats other than HTML, RTF, plaintext, TIFF, PNG, JPEG, PDF, and QuickTime movies, consider providing a Quick Look generator. For developer guidance, see Quick Look Programming Guide.

Report disk usage and file sizes accurately and consistently. Provide values that are consistent with values reported by the Finder and utilities like Activity Monitor. It’s confusing when an app and the system report different values for the same measurement.

Avoid changing the creator types of existing files. A creator type implies a distinct sense of a particular app’s ownership over a file. Your app can assign a creator type to files it creates, but it shouldn’t change creator types for files created by other apps unless the user gives explicit consent. Note that the user can still associate files with a specific app by using the Info window.