Down loaded m4b audiobook file from Learnoutloud.com website. Can only use dropbox, fileapp or evernote to access file. Cannot play. Option at learnoutloud site was to dwnld either mp3 or mp4 (m4b). Mp3 would not work with safari, so selected mp4 option. My belief was m4b was native to iPod/iPad. How can I access already paid for m4b file and play audiobook?