[Doitsu]

Originally Posted by User_Z

Added a pdf document to ePub.
Now, when opening a book, the editor issues a warning about an unknown type.
Sigil doesn't understand basic MIME types?

You'll need to create a fallback item. For more information see this post.
Unfortunately, the current Sigil version doesn't support application/pdf and won't display the fallback .xhtml page.

@KevinH & DiapDealer
(Sigil 0.9.8 shows the fallback text; application/pdf is an IANA mime type.)

Here's a link to an old proof-of-concept test case that'll generate the same warnings that the OP reported.
It passes EPUBCheck, except for an unrelated empty guide item error.

The embedded Calibre sample .pdf file looks like this in ADE 2 and 4:

Spoiler:

Here's another test case by Jellby: ePUB with PDF embedded test