The `-2`, `-3` suffix for a data set output that already exists is now
looked up only when the destination folder and the output path pass the
read and write access checks, so a call cannot learn which files exist in
a folder it may not use.
The lookup uses only extensions of formats the app knows, without path
separators or `..`, and only paths whose parent is the destination folder.
A step whose `format` is unknown keeps its own path instead of being
pointed into the destination folder. A data set or document named `.` or
`..` gets the `Untitled` stem with its three-digit number.
dotCooCoo
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