To explain the actual format of http request more clearly, the first attachment is a snippet excerpted from Fiddler. From the screenshot, you can see each Content-Disposition is actually separately by "--"+boundary. If you need to upload multiple documents, the name of the Content-Disposition shares the same as “file”.
And at the end, you need to use "--"+boundary+"--" to end the request.
And the second attachment is an example for creating a package with multiple documents, but it’s using WebClient object to send the same REST package.