Well you can use ontrack easyrecovery or fixit software for this pupose....I have recover lots of files with these stuff....and I hope you know how to purchase a software over the net.
But one thing you need to know and that is if your file is corrupted for download manager then it will have chance less than 1% to recover or if your file is not corrupted for download manager..I mean for any other like..virus..partition systems etc...then you have the chance of 99% to recover that.
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