On my server as I am trying to get my staff to use it more, a non technical users asked why when she selected a gif that it did not show the gif, it showed just a link. It took some time, but I was able to show her how to use mattermost code to get the image to display, but it did not promote buy in.
My suggestion would be that when gif is selected, the mattermost code is used and it is displayed. In her example, it would have been: