I always use attachments on Agenda, however it just attached as thumbnail, I would prefer it to show the complete attachment in the original size of the window, not sure if you are going to support them in the future?
If you click on the attachment, you can choose different sizes. There is a full width option, which I think is a premium feature.
Note that it won’t go the full width of the window, just the width of the text. If you need to see in detail, you can use the Preview option.