Control Downloaded Traffic

Yes, of course. For example, the free eCAP ClamAV adapter does monitor (and may block) traffic:
http://www.e-cap.org/Downloads

Is it possible to support HTTPS traffic ?

Yes, provided the host application decrypts HTTPS into HTTP.

Without decryption it is only possible to control HTTP CONNECT requests, provided the host application sends those to the eCAP service.

Good afternoon,
those I correctly understand that if
1) For example, a module will be configured on the SQUID
2) To the squid come requests from CONNECT the frontend with termanate SSL
3) the squid itself works without ssl

I can get statistics on incoming / outgoing traffic ?

If you are plugging in your adapter into a proxy that handles unencrypted/plain traffic, then your adapter will have access to incoming (i.e., received by the proxy) and/or outgoing (i.e., sent by the proxy) HTTP messages. In Squid case, your adapter will have access to incoming requests and incoming responses.

The proxy itself also collects statistics. An adapter is only needed if you want to collect statistics that the proxy does not _and_ it is easier to write/support an adapter than to enhance the proxy.

If you need Squid-specific support, please post your questions to the squid-users mailing list:
http://www.squid-cache.org/Support/mailing-lists.html#squid-users