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.
Using: Ubuntu(32 bit):Oneric,Precise and XPpro SP3, W7HP(64)- - Libre Office w/Writer2EPUB