I had an logo I wanted to use for a JIRA project log (basically I just grabbed the logo from our client's web site) which was in .webp format.
When I attempted to upload to image, JIRA validation error prompted me that this format was not recognised.
I renamed the file extension to .png and, despite these formats not being binary compatible (i.e. completely different headers etc.), it worked!
I'm assuming it worked because the file extension fooled the validation logic, and the utility used for creating thumbnails is ignoring the file extension and reading the actual header of the file and already supports the .webp format.
Please can someone change the validation to allow this image format? It's been around for some time now, no reason to leave it out :)