you want to disallow caching of specific file types? it's done in the "/ip proxy cache" menu:
Cache Management
Submenu level: /ip proxy cache
Description
Cache access list specifies, which requests (domains, servers, pages) have to be cached locally by web proxy, and which not. This list is implemented exactly the same way as web proxy access list. Default action is to cache object (if no matching rule is found).
Property Description
action (allow | deny; default: allow) - specifies the action to perform on matched packets
allow - cache objects from matched request
deny - do not cache objects from matched request
dst-address (IP address/netmask) - destination address of the IP packet
dst-port (port{1,10}) - a list or range of ports the packet is destined to
local-port (port) - specifies the port of the web proxy via which the packet was received. This value should match one of the ports web proxy is listening on.
method (any | connect | delete | get | head | options | post | put | trace) - HTTP method used in the request (see HTTP Methods section in the end of this document)
path (wildcard) - name of the requested page within the target server (i.e. the name of a particular web page or document without the name of the server it resides on)
path (wildcard) - name of the requested page within the target server (i.e. the name of a particular web page or document without the name of the server it resides on)
src-address (IP address/netmask) - source address of the IP packet