I'm on version 184.108.40.206.0.
limiting bandwidth on MWG 7.x is only possible on a "per connection" basis. This means anywhere in the rule set you say "this connection should not be faster than 100 kb/s" and MWG will apply this. This works by manipulating the TCP window size, which means it works better the bigger the file is. For very small files the throttling may not be too accurate.
Basically check your log files and find the correct user-agent. Put them into a list and then add a rule at the beginning of the policy that calls the "Throttle.Server" Event when the user-agent matches in the list.
You should keep in mind that a lot of communication is encrypted, which means that MWG may not be able to notice what traffic is mobile traffic and which is not. I have seen customers who built a "separate" wifi network which is only used for personal mobile devices, and has a DHCP which distributes a known IP range. The customers then apply special rules to this IP range.
Hope that gives you some ideas to start.
Since this trafic is coming from your WiFi access, why not do the trafic shaping there (at the interface linking WiFi trafic to Corp Network) instead of at the proxy?
My understanding is we don't quite have that feature yet on our WiFi controllers but it's coming.
Can be done at the wifi controler or in the router/firewall on the network interconnect