If you are running browser extensions that are focused on privacy and limiting tracking, those extensions can sometimes interfere with the proper operation of some web applications, including Discourse. For example, in Firefox I was having problems uploading files to the forum because of an extension I am running called ClearURLs. Details here:
You might want to review the extensions you are using in your browser.
I traced this issue to Privacy Possum Firefox plugin. It seems Steinberg will not allow users to upload images to the forum if they use Privacy Possum protection.