private static List <InvoiceAttachment> ConvertAttachments(string filePath, string folderPath)
        {
            var extension        = Path.GetExtension(filePath);
            var attachmentImages = new List <InvoiceAttachment>();

            switch (extension.ToLower())
            {
            case @".msg":
                attachmentImages = ImageFunctions.ConvertMsgToPng(filePath, folderPath);
                break;

            case @".pdf":
                attachmentImages = ImageFunctions.ConvertPdfToPng(filePath, folderPath);
                break;

            case @".tif":
            case @".tiff":
            case @".jpg":
            case @".jpeg":
            case @".png":
            case @".bmp":
                attachmentImages = ImageFunctions.ConvertToPng(filePath, folderPath);
                break;
            }
            return(attachmentImages);
        }