You can instantiate your controller like this:
ICommandBus commandBus = DependencyResolver.Current.GetService<ICommandBus>();
var fileShareController = new FileSharingController(commandBus);
Generic GetService() method is extension method, so make sure that you have "using System.Web.Mvc;" line in the cs file.
But then, it's better to have helper class that is responsible for keeping/storing already uploaded files, and call it from both controllers, instead instantiating controllers manually.
For example:
public class FileUploadManager
{
public FileUploadManager(ICommandBus commandBus, HttpSessionStateBase sessionState)
{
//....
}
}
and then you call it:
ICommandBus commandBus = DependencyResolver.Current.GetService<ICommandBus>();
var fileShareController = new FileUploadManager(commandBus, this.HttpContext.Session);
Or, if you don't want to use DependencyResolver, you pass ICommandBus to both controller's constructors, and use that reference to instantiate helper class.