My apologies Takasi... I was wrong about the import. I'd tried a number of files... but then forgot to turn Deploy to Target Directory back on. I tried apple.traineddata, apple.train, apple.trn, and apple.htm. Only apply.htm did not cause the issue.
So... I would conclude that OS is only allowing known filetypes. I would report this and see if there is a reason.
I've tested the following:
apple.htm - no issue
apple.jpg - no issue
apple.png - no issue
apple.eps - no issue
apple.hqx - a mac file extension - no issue
apple.class - a valid extension - failed
apple.mtb - a MiniTab extension - failed
apple.html - no issue
