How to update virtual property in EF6 Code First?

I'm trying to update a virtual property using EF6 and MVC. My code goes like this:

public async Task<ActionResult> Edit([Bind(Include = "Id,Title,Content,Image")] Recommendation recommendation, HttpPostedFileBase file)
        {
            if (ModelState.IsValid)
            {
                if (file != null)
                {
                   //we have an image - saving it to db
                    var image = Server.SaveFile(file, Request);
                    DB.Files.Add(image);
                    await DB.SaveChangesAsync();

                    //assign the new image to the recommendation
                    recommendation.Image = image;
                }

                recommendation.User = await GetLoggedInUserAsync();
                DB.Entry(recommendation).State = EntityState.Modified;
                await DB.SaveChangesAsync();
                return RedirectToAction("Index");
            }
            return View(recommendation);
        }

The image is saved in the Files table. Yet the Image_Id column in Recommendations table won't update to the new Image_Id. What am I missing here?

Classes:

     public class Item
     {
          [Key]
          public int Id { get; set; }
          public virtual ApplicationUser User{ get; set; }

         [Required]
         [DataType(DataType.Text)]
         [Display(Name = "Title")]
         public string Title { get; set; }

         [Required]
         [DataType(DataType.MultilineText)]
         [Display(Name = "Content")]
         public string Content { get; set; }
    }

    public class Recommendation : Item
    {
        [DisplayName("Image")]
        public virtual File Image { get; set; }
    }

    public class File
    {
        [Key]
        public int Id { get; set; }        
        public string Name { get; set; }
        public string Url { get; set; }
    }