I'm almost sure isn't a matter of format file; i've tried sending in different ways and nothing happened. In fact, this is what I see when I can choose format (0 of 1 books).
I've also tried different books, no difference.
All my books are saved in 'documents', a folder in main. Calibre placed them there since the beginning.
And, to be honest, I don't know how to check the things you said .