Update: Solution
slarosa reported on this "Files Don't Attach" thread that the only thing that fixed file uploading was:
EXPLICITLY SET the upload tmp dir in php.ini to be THE SAME of the tmp dir set up in the file upload config in Drupal; for instance:
vi /Applications/xampp/etc/php.ini
Then in the file, make sure this is set and not commented out:
upload_tmp_dir = /tmp
That's how to do it.
For historical fun Agaric's misdirected debugging will stay down here.
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