Not quite right. A Kobo ereader needs the .kepub.epub extension to trigger use of the Webkit based renderer. There is also the .fxl.kepub.epub extension for fixed layout epub3 ebooks however that is not really needed if the .opf file properly identifies the epub3 as prepaginated.
My workaround for when I've played with sending kepubs via Dropbox is to use kepubify and a batch file to convert the books after copying them to the folder. Perhaps the OP could use something similar.
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