There are usually two common reasons this can happen. The first is that you are attempting to build an icon for Mac OS X that only contains the 128×128 thumbnail resource. There is a bug in certain versions of Mac OS X that will cause the icon to show up as a generic document icon unless you include a 32×32 resource as well. The solution to this is to make sure you build in the 32×32 (and 16×16 for good measure) resources.
Another possible explanation is that you are saving out the icon file out as ”.icns” instead of standard Macintosh resources. ICNS files are provided for developers and will not show up on the Macintosh desktop.
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