not sure what you are asking for here. I'm a user not a mind reader

Then read what I wrote. I spelled it out. It's impossible to upload images when the expectation set by "new file" is that you should be able to. Nothing says that you can't.

Nothing says that you can't.

Because you should be able. Its a bug.

The way that you wrote it suggested that you want a better error message to say that you cant upload an image.

The problem is not the error message. The problem is a bug in joomla preventing you from uploading

And its not actually a bug.
The problem comes from when the webp support was added. This required a data change in the db. You will not get those changes applied unless you do an upgrade or clean install. The database structure fix cant change data. OR open the template options and press save. Any of those will resolve the "bug" and you can happily upload images.

I tested that theory - I reinstalled from scratch... and now I still cannot upload png files (screenshots)

However, the message is complaining about "not being a valid file name", although this is a valid file name, Im guessing Joomla doesn't like the multiple periods in the file name.

Screenshot 2021-05-09 at 11.32.29.png

I can upload test.png

Reopening as Screenshot 2021-05-09 at 11.32.29.png file can be uploaded with com_media, so it should be legal and valid to upload it using template manager too.

You're right. I forgot about the double dot. That is a restriction put in place several years ago for security

The file uploads perfectly well in the Media Manager, and so should be allowed here too.

Or blocked in both if the security issue is still valid

Related issue #30908

In J3 you do get the correct error message

That is the same error message as joomla 4