How to implement multiple files selection and upload using HttpFileCollectionBase in ASP.NET MVC3?

Assuming the name of the file input is for example files you could have the following controller action:

[HttpPost]
public ActionResult Upload(IEnumerable<HttpPostedFileBase> files)
{
    ....
}

or define a view model:

public class MyViewModel
{
    public IEnumerable<HttpPostedFileBase> Files { get; set; }
}

and then:

[HttpPost]
public ActionResult Upload(MyViewModel model)
{
    ....
}

Once again it is important to have the file input named the same. For example:

<input type="file" name="files" />
<input type="file" name="files" />
<input type="file" name="files" />
...

UPDATE:

It appears that you want to use the PLUpload plugin. This plugin sends each file individually and uses chunks. So here's an example of how you could handle the upload on the server:

[HttpPost]
public ActionResult Upload(int? chunk, string name)
{
    var fileUpload = Request.Files[0];
    var uploadPath = Server.MapPath("~/App_Data");
    chunk = chunk ?? 0;
    using (var fs = new FileStream(Path.Combine(uploadPath, name), chunk == 0 ? FileMode.Create : FileMode.Append))
    {
        var buffer = new byte[fileUpload.InputStream.Length];
        fileUpload.InputStream.Read(buffer, 0, buffer.Length);
        fs.Write(buffer, 0, buffer.Length);
    }
    return Json(new { message = "chunk uploaded", name = name });
}