Validating Uploaded Files Using IValidatableObject - image

Question

Hi All!
I'm a bit of a noob at model validation and i've been trying to validate an Articles object and an uploaded file using the IValidatableObject interface with no success.
This following class validates the Articles object just fine but I can't see how the HttpPostedFileBase is injected to allow me to validate against it. Is this even possible to achieve using this method?
The form i'm using to submit the data includes the enctype = multipart/form-data attribute so it knows its posting files.
This is the full class im trying to validate. This ones really got me stuck and any help will be very gratefully appreciated.
public class ArticlesModel : IValidatableObject
{
public Article Article { get; set; }
public IEnumerable<Category> Categories { get; set; }
public HttpPostedFileBase PostedFile { get; set; }
public ArticlesModel(){}
public ArticlesModel(Article article, IEnumerable<Category> categories)
{
this.Article = article;
this.Categories = categories;
}
public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
if (Article.CategoryID == 0)
{
yield return new ValidationResult("Please select a category.", new[] { "Article.Category"});
}
if (Article.Title == null)
{
yield return new ValidationResult("Please enter a title.", new[] { "Article.Title" });
}
if (Article.Content == null)
{
yield return new ValidationResult("Please enter some content.", new[] { "Article.Content" });
}
if (PostedFile == null)
{
yield return new ValidationResult("Please upload a file.", new[] { "Article.ImageFile" });
}
else
{
if (PostedFile.ContentLength > 1 * 1024 * 1024)
{
yield return new ValidationResult("Please upload a file 1Mb or less.", new[] { "Article.ImageFile" });
}
//Other file checking logic here please!!
}
}
}