Returns the resume data for a download that is not yet complete.
- macOS 10.4+
The resume data for a download that is not yet complete. This data represents the necessary state information that an
NSURLDownload object needs to resume a download. The resume data can later be used when initializing a download with
nil if the download is not able to be resumed.
Resume data is returned only if both the protocol and the server support resuming. For details on how to resume a connection, see the documentation for