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I have seen books where there were deliberate errors in the opf with file extensions. Somehow the book worked fine on most readers, but upon opening in Sigil not everything was loaded. The reason is logical, Sigil seems to use the opf to load all the files. So, if a file has a different name, it is not loaded into Sigil.
Perhaps this is also such a case where a deliberate error/loophole is exploited. Have you checked the original file with EpubCheck?
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