If your sources tend to contain files that are not of use to you, filter them by specifying their file extensions. If you want to be really strict, for example when importing a Sony card when you might not want anything but the media files copied, you can do the opposite: ignore all files, except the ones defined, by switching from
Do not copy files to
Copy only files . Use the latter option with caution, as it's easy to forget you have this setting applied. Pro tip: only use it as part of a Preset.