Resume an Accidentally Removed/Missing Transfer in Opera

This tip will help you to resume a download (especially large size) in the following cases.
Accidentally removed transfer from the transfers tab.
Closed Opera with an intention to resume download next day but transfer is missing when Opera is started.
Resume download using another mirror.

1. Rename the downloaded part
2. Start to download the original file again
3. Stop the new download a few seconds later
4. Quit Opera
5. Replace the new download file with the old one (simply rename the old file to the original filename)
6. Start Opera
7. Opera should be able to continue the old download

Source