Just as I thought... OP's app doesn't really do the conversion part on Android device/using it's hardware, it *just* uploads a file to some remote server with Calibre (v2.49.0
) installed that converts the book and then downloads it back to Android device and uses existing e-mail app to send the book. At least it's not using totally obsolete version of Calibre like 90% of "online converter" websites
and it's convenient & n00b friendly.
@iaroslav: Since you're already using Calibre it would be nice if future versions of eBooker could add option in app settings to output "dual mobi" file (=mobi output> file type "both" in Calibre conversion settings) instead of "old mobi" ... so it can be accepted by Amazon's personal documents service (which doesn't accept books with "mobi new"/"KF8/azw3" only formatting) but delivers .azw3/KF8 formatted book to supported devices like Paperwhite.