Add support for viewing webp images

Description

I've recently switched from PNG to WebP in my project.

When viewing the diff of a MR, it will show PNG-files but not WebP-files ("No preview for this file type").

Proposal

Enable showing WebP-files just like any other image. It should be no problem for modern browsers to display it, right.

Links / references

• https://developers.google.com/speed/webp/