Originally Posted by theducks
I think you are using the wrong menu choice (unless MTP works way different than Save to Disk).
You should use the normal 'Send', that way the list works without conversion. If you choose to send a Specified format, it MUST convert if it does not exist. The rest of the device valid formats are locked out by your choice.
I have been using the normal "Send to device" button. However, I did look at the drop down menu and saw the entry "Set default send to device action". I selected this and then "Send to main memory" (the only place it can go a Nexus 7). Now it seems to work like I expect. I can send mixed-format batches of books so long as all of the formats are acceptable to the device. I guess the default action had previously been "Send specific format to device". There does not seem to be any way to tell, at a glance, what the current default action is, which only added to my confusion.
Thank you for your help.