Configure the device plugin to list EPUB on top, and MOBI as second option. It will send EPUB if available, and fall back on MOBI if EPUB is not available. If neither is available, it will ask you whether you want to convert the non-available books to a device-supported format.
If the book is being auto-converted,
this is wrong!!! It means you have accidentally set calibre to automatically do that in the future. Go to Preferences ==> Interface ==> Behavior and use the tool to "Reset all disabled confirmation dialogs".
Now you will be prompted, instead of calibre automatically assuming you want them converted.
