I am supposed to dress down to show my brain

yesterday’s work experience left me speechless and I don’t know what to do. I work in the IT industry and changed a couple of month ago my department. In my new department I am allowed to go to conferences and also speak in front of the audience. I hold a Master in Engineering and I have never been afraid to speak up. I am known in my company to do so that’s why the new department also wanted me. So yesterday my boss talked to me telling me when I go to conferences, I should tone down my dressing. Normally I wear sheath dresses and heels and obviously this isn’t appropriate. The reason? I should be recognized for my brains and not my looks. I am so mad. I told him once I open my mouth, people know I got brains. He just said in the company you can wear whatever you want, people know you but for conferences I should dress with sneakers and a nice blouse to match the audience. This left me speechless. I should add that I have a large chest and it would probably draw more attention to my looks again than wanted (that’s why I wear dresses…) Any advice you would give to me how to deal with it? And I appreciate a supplier for potato and coffee sacks, maybe I can wear those *irony off