Sometimes you need to manually resume a download in Safari, but you can’t, because the download url was part of a session that has expired.
Mac OS X hints has a handy tip on how to resume a download after Safari crashes – Resuming a download after Safari crashes.
With a slight modification to that process, you can manually resume a download that requires a session, for example by requiring you to log in to a website.
Follow the same steps as in the hint above, and after starting and stopping the download, copy the following two items (substitute the urls from the info.plist in the new download file) to the info.plist in the old download that you want to resume –
Now open the old download package in Safari, and it should be able to continue the download from where you left it.