Submitting a Stream as part of a JSON body

I have a PATCH method in an REST MVC controller implemented in C# that I need to be able to call. The body of the PATCH request is an object (received as JSON) where most properties are standard types but one (which is a file) is of type Stream.

Like this:

public class MyDTO{    public string SomeProperty { get; set; }    public Stream SomeFile { get; set; }}

When running this service the Swagger UI exposes SomeFile as a string in the example JSON it generates.

When I am writing a client for this I assign values to these properties, I can assign a Stream value to SomeFile generated from an actual file from the filesystem. However when I try to run this I get the error

Timeouts are not supported on this stream

I have tried copying the FileStream to an in memory stream but this has the same problem.

How do I assign a value to this property to call the service?