I was fixing a bug in OmniOutliner where it wouldn’t open a file with an uppercase .OPML suffix. I did some digging, and the fix was to register the app as handling the com.apple.news.opml file type.
OPML — Outline Processor Markup Language — was invented in 2000 by Dave Winer at UserLand Software. It’s not Apple’s format, and the correct file type is org.opml.opml.
I seem to recall a similar issue with a non-standard UTI for Markdown.
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