How can I turn off Finder's icon preview for my app's document type?

Question

I have managed to do the proper dance and get a custom UTI set up for my OS X app's document type, with a custom file extension and so forth. I set up an icon for my file type as well, and that is also working. I know this because when I save a new file from my app, I can see it appear in the Finder with the correct icon. BUT – about a second later, the Finder replaces my icon with one of its generic "preview" icons, showing the text contents of my file inside the icon, with my file extension, capitalized, at the bottom of the preview icon, like this:

I think this is happening because I declare my custom UTI as having MIME type text/plain and conforming to public.utf8-plain-text. I do want to do that, because they are in fact text files, and I want to allow the user to treat them as such, other apps to recognize them as such, etc. But given this, the Finder (perhaps through some QuickLook generator or something) therefore says "aha, it's a text file, I know how to make custom preview icons for text files!" and replaces my custom icon. Which, needless to say, is not what I want. How can I prevent this from happening? I'm using Objective-C in my project, but I don't think this is a language-specific issue; it all has to do with the UTI declarations in my app's plist.

Speaking of which, here's a screenshot of the way my UTI is configured in Xcode:

And here's a screenshot of the UTI export in Xcode:

It is fine that the icon previews there show as question marks; that is because the icon images are included in my project as a .iconset folder that is turned into the requisite .icns file by Xcode at build time, and Xcode isn't smart enough to show the right preview. But as I said above, the correct icon does display for a second in the Finder, before it gets replaced by the preview icon, so the setup of the icon .icns file and the binding of the .icns to the UTI is clearly working. Along the same lines, if I do a "Get Info" on my file in the Finder, by the way, it shows the correct icon there in the upper left of the information window. The correct icon is also shown in the title bar of the window in my app. It is just in the Finder's display that the wrong icon is used. Any ideas?

Answer 1

Upon further experimentation, the best solution I've been able to find so far involves changing the "conforms to" field from public.utf8-plain-text to simply public.text. The public.text UTI is for text of any kind – any encoding, with or without markup, etc. Any more specific conforms-to UTI seems to lead to the Finder (or whoever it is) replacing my icon. But with public.text the icon is not replaced, presumably because whoever is doing this feels like they don't have enough information to proceed, since public.text is so generic.

This is really non-optimal, since it means that the encoding used for my files is not stated publicly in my UTI export info, making it less likely that other apps will be able to open my app's files as text. If anybody else has a better solution, I'd be happy to vote for it in preference to this answer. But for posterity, I've put this answer up since it might help someone else in a similar boat.