Hello Emanuele! I am copying my reply here as well so other users/administrators can benefit from the follow up.
Just a quick follow up on this for you as someone here read my reply and discussed this further with me. Based on what I've had described to me, I need to correct my previous reply!
The plain text default here was purposefully implemented so that the ticketing interface can display as much of the pertinent detail as possible to the agent using it, and is similar to how email applications display a message preview (in plain text) consisting of just a few lines. This allows the agent to complete a high level review, then dig into it further if necessary.
If the full message is required the agent should then use the Show HTML option to open the original message including its HTML and other content. This was a design choice in order to maximize the screen real estate usage for critical details and there are no plans at this time to modify this design. That said, I will certainly still forward your feedback regarding a default HTML view. I just wanted you to be aware of the full scope for the reason it was designed in this way. Please let me know if you have any questions on this. Have a good one!
Technical Support Specialist