[KevinH]

I took a look at your opf but did not see any media-types missing or incorrect. So unless there is a non-obvious typo in them, I did not see a problem.

But if you use the latest Sigil and hit the details button on the warning message dialog when it comes up and copy exactly what it says (or screencap it) and post it here. That will tell you everything you need to know on what to fix in the opf.

The newer version of Sigil will tell you which file, what the wrong media type was, and what it chose to use instead.

But as I said, I could not see any obvious error when I opened your OPF. I do not have access to Sigil to test it and report back as I am travelling today.

Could you have added an unmanifested file to your epub somehow?

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Update: dug out my old laptop and fired up your linked epub, and there were *no* warnings at all when opening it with Sigil-2.5.2

So I have no idea why you are seeing any warnings at all when editing that epub in Sigil.