Returns a URL that opens a preview page for the file identified by hash.
The path argument is informational, but its extension must match the file
type (for example, .csv or .ion).
hash: the file hash in Lokad storage.
path: the file path or name to display in the preview.
Example
hash = "0123456789abcdef"
url = previewUrl(hash, "/data/items.csv")
show scalar "Preview url" with url
Remarks
This function requires a valid file hash from the Lokad file storage, and it
cannot be executed on try.lokad.com.