Now it is clear.

We have no relation to download manager.

You download the video through the browser, and the browser is already changing the name.

In what capacity do you download the video?
How exactly do you download the video?
Describe in more detail.
Please send screenshots of the error.

Hello, reinstall the extension: http://download.sf-helper.com/mozilla/helper_last.html

Clear the browser cache.

If you start the video arrow appears?

Hello, the information how to install the extension to Google Chrome browser
can be found here: http://en.savefrom.net/faq.php#ext_inst_chrome

Hello, the name of the site from which you want to download the audio?

Hello, Unfortunately, we don't fully understand you... please. provide us with more details regarding the issue.

Hello! Our help will be more effective, if you copy and send us info about your browser and the Savefrom.Net extension: 
http://en.savefrom.net/user.php?info=1 
For the better understanding it'll be great to see a screenshot with the error.

Reinstall the extension, remove the Chameleon extension.
How to install the extension to Google Chrome browser read here: http://en.savefrom.net/faq.php#ext_inst_chrome

Clear the cache and cookies in your browser.