By default, if you do not specify a document icon for a file type in your app, macOS will automatically create one for you by composing your app icon along with the correct extension name. This is a common pattern for imported document types or non-proprietary file formats that your application can open, such as MP3, JPG or PNG.[…]
To create a new custom document icon, you can specify a background fill, center image, or text string. Each of these three elements is optional, so you can use just one element or any combination of the three to customize your icon. From there, macOS will automatically store, place and mask these elements, and then assemble them into a page icon with a fold in the right corner.
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