Your extender is WA830RE? The firmware is up to date?
Can you connect to LAN port of the extender and try to download the file?
Did you test and compare the speed connecting to the root router directly and connecting to the extender?

yes, my extender is the TL-WA830RE V2, the firmware is up to date. I tried download the file via LAN port in the extender, but the error continues. I think there is no problem with the connection speed, when I download the file connected with the root router, the kb/s are the same in both devices.

I´ve noticed that when the download fails, the internet connection is lost for a little time, then the internet connection returns (a yellow exclamation appears next to the wifi icon in the windows bar), this must be the main problem I have, the internet connection is lost momentarily stopping the current downloads.

830RE_V2 has been sold for quite long time, it's nearly impossible to have a so severe BUG in the firmware (or it'll be stopped in the market..).
Maybe the one you're using has got some problem, when did you buy it? If permitted, it's suggested you ask the shop where you bought it to provide you a replacement.
If it has exceeded the time limit, you can contact TP-LINK tech support to help you: http://www.tplink.com/en/support-contact.html (you can choose Hotline support or E-mail support)