I'd like to use Umami Analytics' new Pixels feature to generate small images that can be embeded in our emails to track when they're viewed. However, using Proton Mail's "Insert image" option it gives me an error "Not a valid URL". Which I'm not sure if it has to do with the type of image Umami generates (a GIF) or if it's due to the URL missing a file extension.

For reference, you can inspect the Pixel here and see for yourself how Umami implements the image: https://cloud.umami.is/p/1i49B6ecn