How can I prevent itunes to sync mp3 files in downloads folder?
Follow these steps to prevent your device from syncing automatically.

Open iTunes Preferences and select the Devices tab.

To prevent iPod, iPhone, and iPad devices from syncing automatically, select the option below:
"Prevent your device from syncing automatically"


After you select this option, reconnect your device.

You can than select music you do want to sync. You can also use smart folders to sync selective music.

source support.apple.com