Took me a few days, but I have found a solution!

It's not the most elegant and there is one downside - Once a file is uploaded you can replace it, it will always be saved, but you cannot remove the current file completely... maybe someone has a solution to this, but for my purposes it works!

Basically if a value exists I create a session with the key being the GUID and the value being the URL in the view...

@using Umbraco.Web
@model Umbraco.Forms.Mvc.Models.FieldViewModel

<input type="file" name="@Model.Name" id="@Model.Id" multiple
@if(Model.Mandatory){<text> data-val="true" data-val-required="@Model.RequiredErrorMessage"</text>} />

@if (Model.Values != null && Model.Values.Any())
{
    Session.Add(Model.Id, Model.Value); // <-- HERE!

    <p>
        <strong>Current File/s:</strong><br/>
        @foreach (Dictionary<string, string> dictionaryObject in Model.Values)
        {
            @*
            The model.values contains a list of dictionary objects with a single item
            With the key being the full path to the file
            And the value being the filename
            *@
            if (dictionaryObject.Count > 0)
            {
                var fileInfo = dictionaryObject.First();
                <a href="@fileInfo.Key" target="_blank">@fileInfo.Value</a><br/>
                <input type="hidden" name="@(Model.Name)_file_@(fileInfo.Value)" value="@fileInfo.Key" />
            }
        }
    </p>
}

Now all I need to do is handle that value inside the 'ProcessSubmittedValue' method....

private List<Object> myValue = new List<object>();

public override IEnumerable<object> ProcessSubmittedValue(Field field, IEnumerable<object> postedValues, HttpContextBase context)
{
    string fieldFolder = field.Id.ToString();
    string newFolder = DateTime.Now.ToUniversalTime().ToString("ddMMyyyyhhmmss");
    string dir = "/media/forms/" + fieldFolder + "/" + newFolder + "/";

    IList<HttpPostedFileBase> files = context.Request.Files.GetMultiple(field.Id.ToString());

    foreach (HttpPostedFileBase file in files)
    {
        if (file.FileName != "")
        {
            // Create directory
            if (!Directory.Exists(HttpContext.Current.Server.MapPath(dir)))
            {
                Directory.CreateDirectory(HttpContext.Current.Server.MapPath(dir));
            }
            // Save the file
            file.SaveAs(HttpContext.Current.Server.MapPath(dir + SafeUrl(file.FileName)));

            // Add string to the form       
            myValue.Add(dir + SafeUrl(file.FileName));

            // Remove any old file
            if (context.Session[field.Id.ToString()] != null)
            {
                string oldFile = context.Session[field.Id.ToString()].ToString();
                string oldDir = oldFile.Substring(0, oldFile.LastIndexOf("/"));
                // Remove old directory
                if (Directory.Exists(HttpContext.Current.Server.MapPath(oldDir)))
                {
                    Directory.Delete(HttpContext.Current.Server.MapPath(oldDir),true);
                }
            }
        }
    }
    // If nothing has been added but a previous value exists in session add to the list and remove the session! 
    if (myValue.Count() == 0 && context.Session[field.Id.ToString()] != null)
    {
        myValue.Add(context.Session[field.Id.ToString()].ToString());
    }
    // Clean up session
    context.Session.Remove(field.Id.ToString());

    return myValue;

    // NOTHING TO BE DONE
    //return base.ProcessSubmittedValue(field, postedValues, context);
}

And that works for me! High five if I saved you a few days of puzzling this one out :-)