Chat with External Users does not allow to paste pictures and other things.

Hello Everyone,

I have added an external domain to allow communication with our company Teams. Searched, found and added the external user. Where it appears for me and for him with a little Skype Icon under the name even thou we are both using Teams. Besides this, we do not have any functionality of a normal chat. I can't paste an image on the chat per example. 

Is this annoying thing by default? What is the solution for this?

Thank you all in advance.