Thanks again for posting. @StuM definitely makes a good point – extensions can interfere with actions in the Designer.
One other thing to note, is that if these assets exist in the Asset manager already, the same file with the same file name will not upload a second time. The Asset manager will recognize these duplicate images and cancel the upload.
If you need to upload resized, or other variants of existing files, you can delete the original and upload the new one, or you can give the variant a new name.
We tested on this end and it looks like we can upload images in a new project and in a copy of your project without issues. I’m standing by for more information