why this code is showing the error as the name uploader could not exists in current context?

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ASP.NET

C#5.0

so basically i wrote code for importing excel and saving it on godaddy domain but if i try to run this code it shows me error the uploader could not exists in current context what m missing here?
code is:

private String uploadAttachmentOnServer(FileUpload fileUpload)
    {
        String fileName = null;
        int filesize = 0;

        try
        {
            fileName = fileUpload.FileName;
            filesize = fileUpload.FileBytes.Length;
            String fileType;
            String fileSize;
            String destinationPath = Server.MapPath("./ExcelFiles/"); // ExcelFiles is folder name

            String extension = Uploader.fileUpload(fileUpload, destinationPath, out fileType, out fileSize);
            String fileVirtualPath = null;
            if (extension != null)
            {
                fileVirtualPath = "../ExcelFiles/" + fileUpload.FileName;
                return destinationPath + extension;
            }
            return null;
        }
        catch (Exception ex)
        {
            throw ex;
        }
        return null;
    }