When you upload or download images, you may receive the following error message returned by the ErrorString property:
1. The problem may occur when the file type is not specified.
2. Dynamic Web TWAIN does not support the image type. Currently, Dynamic Web TWAIN supports the following types of image files: BMP, JPEG, TIFF, PNG and PDF.
Please add the file extension to the file name when uploading and downloading images. For example, if you want to download a JPG file using the HTTPDownload method, please don't forget to add the extension .jpg.