You could try something like:
AttachmentInfo attachmentInfo = DocumentHelper.GetAttachment(documentGUID, tree, siteName, true);
if (attachmentInfo != null)
{
string fileExtension = attachmentInfo.AttachmentExtension;
filePath = VirtualPathUtility.ToAbsolute(AttachmentURLProvider.GetFilePhysicalURL(siteName, documentGUID, fileExtension));
}