Medium.com does not support image file format .webp
For a long time the common supported file format for compressed colour images was JPEG, file extension .JPG or .jpg. But times have moved on, and a lot of web sites nowadays support .webp. In general this produces smaller file sizes for the same quality image as JPEG. WebP can be used for both lossy and lossless images. A google guide to this is here
The writers editing tool for medium.com does not allow uploading WebP images. This is a shame, because all the common PHP image libraries, and probably for other web programming languages as well, have fully support WebP already for a number of years. I even updated my own web site framework to support WebP, alongside other older file formats of GIF, JPG, PNG. There wasn’t much code that needed to change, although its a sprawling lot of script. All modern browsers handle WebP images just fine.
Also I was surprised, regarding uploading .JGP or .jpg that the interface won’t accept the upper case spelling of the file extension, even though the page specifically says it takes .JPG in upper case! Surely the software can do to lower-case translation for file extension names.
Medium.com ought to give some poor programmer a bit of income, and polish up it its interface capabilities to keep up with evolving web standards, to show it is capable of software maintenance.
I have no other complaints about the medium.com writers interface at the moment.