These files must be "exported" to be useful. The export process
is controlled by the --dropbox-export-formats flag and the ancilliary flags
--dropbox-skip-exports and --dropbox-show-all-exports modeled on the
Google drive equivalents
Before this change rclone would upload the whole of multipart files
before receiving a message from dropbox that the path was too long.
This change hard codes the 255 rune limit and checks that before
uploading any files.
Fixes#4805
