The method you are using only sets the value of the "umbracoFile" property on the new media node (which is a string path to the file on the disk). It doesn't actually try to save the file iteself.

You could in theory manually try to save the file to the disk, then get the path of the file and set that as the value for "umbracoFile". But I dont think that is recommended.

There is an extension method that does all the necessary saving of the file creating random path names, setting the path on the media etc. It takes the file as a Stream. Its not very pretty because it takes a bunch of other services as parameters too. So you would need to inject those services and convert your posted file to a stream with something like this:

using Umbraco.Extensions;

Inject the following services into your controller:

    private readonly MediaFileManager mediaFileManager;
    private readonly MediaUrlGeneratorCollection mediaUrlGenerators;
    private readonly IShortStringHelper shortStringHelper;
    private readonly IContentTypeBaseServiceProvider contentTypeBaseServiceProvider;

Then convert your posted file into a stream and call the extension method something like this (i dont use posted files so this is untested, but it works for me when I get streams from elsewhere)

    using (MemoryStream stream = new MemoryStream(model.myfile.FileBytes))
    {
        media.SetValue(mediaFileManager, mediaUrlGenerators, shortStringHelper, contentTypeBaseServiceProvider, Constants.Conventions.Media.File, model.myfile.FileName, stream);
    }