You are opening a file with "opdownload" extension. It is a temporary file used by the web browser to mark the download as incomplete until it is finished.
Partial downloads are due to the fact that either the file is still being downloaded by the browser or that the download process was interrupted and so it's only a partial, incomplete file.
If the browser is still downloading the file, please wait until the download has been completed by the browser, and the file will be renamed with the proper file extension.
But If your browser download was interrupted before it was finished, you should delete the file and download it again.