Why does the ISingleResult return an array of bytes as a string?

[Ronald Ray]

I need the stored procedure to return the array of bytes instead of a string.

The error I am getting is: Unable to cast object of type System.Byte[] to type System.String.

        ISingleResult<spVTGetAttachmentSelectResult> Result = ava.GetAttachment(AttachmentID);

        foreach (spVTGetAttachmentSelectResult Pdf in Result)
        {

            System.Text.UTF8Encoding Str = new System.Text.UTF8Encoding();
      
            string FileName  = Pdf.Attachment;
       
            string Extension = (Pdf.FileExtension).ToString();
                        
            byte[] DocBuffer = Str.GetBytes(FileName);

        }

The stored procedure:

ALTER PROCEDURE [dbo].[spVTGetAttachmentSelect] (@AttachmentID int)
AS
BEGIN

	SELECT     VehicleAttachments.Attachment, VehicleAttachments.FileExtension
               
	FROM       VehicleAttachments
	
	WHERE     (VehicleAttachments.AttachmentID =  @AttachmentID)	
	
END

[Plater]

Is spVTGetAttachme ntSelectResult a custom class somewhere?
The problem could be with it.
I've never used ISingleResult before so I'm not sure how it is supposed to behave.

EDIT: Scratch that, is the error coming from this line:
string FileName = Pdf.Attachment;

Pdf.Attachment is probably already a byte[]