Add `uuidstamp` env var for `curl-api-upload-file`
Add `uuidstamp` env var for `curl-api-upload-file`

- adds a UUID comment to each uploaded file to ensure uniqueness

E.g.:

```sh
$ photos=( ~/sample-photos/pool/0001.png{,}{,,,,} )
 # photos = ten copies of the same image file
$ curl-api-upload-file $photos
 # uploads 0001.png
 # uploads 0001.png again, which will fail as a dup
 # uploads 0001.png again, which will fail as a dup
 # ...
$ uuidstamp=1 curl-api-upload-file $photos
 # uploads 0001.png with {{uuid1}} as its comment
 # uploads 0001.png with {{uuid2}} as its comment
 # uploads 0001.png with {{uuid3}} as its comment
 # ...
```

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