如何验证IEnumerable使用数据注释

我遵循本教程在HttpPostedFileBase上创建验证，如果我使用HttpPostedFileBase，它可以工作，但如果我更改为IEnumerable<HttpPostedFileBase>上传多个文件，并提交表单ModelState.IsValid总是假的。我上传了。png文件，大小是914字节。如何使用数据注释来验证多个文件上传?

我的模型

public class BillingViewModel
{
     [Required]
     public long BillingID { get; set; }    
     public IEnumerable<TimeKeeper> TimeKeepers { get; set; }    
     [Required]
     [ValidateFile]
     public IEnumerable<HttpPostedFileBase> PostedFiles { get; set; }
}

ValidateFile.cs:

public class ValidateFileAttribute : ValidationAttribute
    {
        public override bool IsValid(object value)
        {
            int MaxContentLength = 1024 * 1024 * 3; //3 MB
            string[] AllowedFileExtensions = new string[] { ".jpg", ".gif", ".png", ".pdf" };
            var file = value as HttpPostedFileBase;
            if (file == null)
            {
                return false;
            }
            else if (!AllowedFileExtensions.Contains(file.FileName.Substring(file.FileName.LastIndexOf('.'))))
            {
                ErrorMessage = "Please upload Your Photo of type: " + string.Join(", ", AllowedFileExtensions);
                return false;
            }
            else if (file.ContentLength > MaxContentLength)
            {
                ErrorMessage = "Your Photo is too large, maximum allowed size is : " + (MaxContentLength / 1024).ToString() + "MB";
                return false;
            }
            else
            {
                return true;
            }
        }
    }

var file = value as HttpPostedFileBase;

var files = value as IEnumerable<HttpPostedFileBase>;

[Required]
public int? ListCount
{
 get {return PostedFiles == null || PostedFiles.Count()==0? (int?)null: PostedFiles.Count();}
}