Solution 1:

MVC 6 used another mechanism to upload files. You can get more examples on GitHub or other sources. Just use IFormFile as a parameter of your action or a collection of files or IFormFileCollection if you want upload few files in the same time:

public async Task<IActionResult> UploadSingle(IFormFile file)
{
    FileDetails fileDetails;
    using (var reader = new StreamReader(file.OpenReadStream()))
    {
        var fileContent = reader.ReadToEnd();
        var parsedContentDisposition = ContentDispositionHeaderValue.Parse(file.ContentDisposition);
        var fileName = parsedContentDisposition.FileName;
    }
    ...
}

[HttpPost]
public async Task<IActionResult> UploadMultiple(ICollection<IFormFile> files)
{
    var uploads = Path.Combine(_environment.WebRootPath,"uploads"); 
    foreach(var file in files)
    {
        if(file.Length > 0)
        {
            var fileName = ContentDispositionHeaderValue.Parse(file.ContentDisposition).FileName.Trim('"');
            await file.SaveAsAsync(Path.Combine(uploads,fileName));
        }
        ...
    }
}

You can see current contract of IFormFile in asp.net sources. See also ContentDispositionHeaderValue for additional file info.

Solution 2:

There is no HttpPostedFileBase in MVC6. You can use IFormFile instead.

Example: https://github.com/aspnet/Mvc/blob/dev/test/WebSites/ModelBindingWebSite/Controllers/FileUploadController.cs

Snippet from the above link:

public FileDetails UploadSingle(IFormFile file)
{
    FileDetails fileDetails;
    using (var reader = new StreamReader(file.OpenReadStream()))
    {
        var fileContent = reader.ReadToEnd();
        var parsedContentDisposition = ContentDispositionHeaderValue.Parse(file.ContentDisposition);
        fileDetails = new FileDetails
        {
            Filename = parsedContentDisposition.FileName,
            Content = fileContent
        };
    }

    return fileDetails;
}