The problem is that calibre does not recognize those formats as "books". CC gets its list of acceptable formats from calibre.
From time to time I (we) have added non-calibre formats to CC's list, with the result that CC and calibre disagree on what formats are valid. In addition, we now have the problem that if we add a format to one version of CC (e.g., Android) then the other version will be different. (Yes, Android and iOS CC are maintained by different people who have *very different* time constraints.). As such, I am not willing to add more "random" formats to CC itself.
However ...
I am willing to change the wireless device driver in calibre to permit a user to add "extra" acceptable formats (extensions) to be sent to CC. I know that this works in Android CC (I just tried it) and I strongly believe it will work in iOS CC. I will test the change a bit more then submit it to Kovid for the next calibre release.
Using this scheme, to add a new format, a user must
- tell calibre about the non-standard formats by opening the "SmartDevice" (the wireless device driver) customization dialog. (I will add a CC FAQ explaining how to open this dialog.)
- connect as a wireless device so CC will be informed about these new formats. Disconnect.
- in CC's Settings / Formats and folders, select that format(s) as one(s) that CC can receive, setting the priority by choosing its position in the list.
- Connect to calibre and transfer books.
This is how we should have solved the problem from the beginning.