added "3.12 Why do FTP specific features over HTTP proxy fails?"

docs/FAQ

-Updated: August 23, 2001 (http://curl.haxx.se/docs/faq.shtml)
+Updated: October 23, 2001 (http://curl.haxx.se/docs/faq.shtml)
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@@ -36,6 +36,7 @@ FAQ
   3.9 How do I use curl in PHP, Perl, Tcl, Ruby or Java?
   3.10 What about SOAP, WebDAV, XML-RPC or similar protocols over HTTP?
   3.11 How do I POST with a different Content-Type?
+  3.12 Why do FTP specific features over HTTP proxy fails?
 
  4. Running Problems
   4.1 Problems connecting to SSL servers.
@@ -375,6 +376,18 @@ FAQ
 
         curl -d "datatopost" -H "Content-Type: text/xml" [URL]
 
+  3.12 Why do FTP specific features over HTTP proxy fails?
+
+  Because when you use a HTTP proxy, the protocol spoken on the network will
+  be HTTP, even if you specify a FTP URL. This effectively means that you
+  normally can't use FTP specific features such as ftp upload and ftp quote
+  etc.
+
+  There is one exception to this rule, and that is if you can "tunnel through"
+  the given HTTP proxy. Proxy tunneling is enabled with a special option (-P)
+  and is generally not available as proxy admins usuable disable tunneling to
+  other ports than 443 (which is used for HTTPS access through proxies).
+
 4. Running Problems
 
   4.1. Problems connecting to SSL servers.