Throw 'UnsupportedOperation' error if unable to set the files with 'input.setFiles' command.

Author: lutien

index.bs

@@ -10526,6 +10526,11 @@ The [=remote end steps=] given |session| and |command parameters| are:
 1. Otherwise, [=update the file selection=] for |element| with |files| as the
    user's selection.
 
+1. If, for any reason, the remote end is unable to set the [=selected files=] of |element| to the files with paths given in |files|,
+   return error with [=error code=] [=unsupported operation=].
+
+   Note: For example remote ends may be unable to set [=selected files=] to files that do not currently exist on the filesystem.
+
 1. Return [=success=] with data null.
 
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