Thank you for the link but the configuration didn't work for me.
I had a look at the source code in webapps/share/components/upload/file-upload.js
In older versions the priority was: flash, html5, html
<javascript>
            // Create the appropriate uploader component
            var uploadType;
            if (this.hasRequiredFlashPlayer)
            {
               uploadType = this.options.flashUploader;
            }
            else if (this.browserSupportsHTML5)
            {
               uploadType = this.options.dndUploader;
            }
            else
            {
               uploadType = this.options.htmlUploader;
            }
</javascript>
While in 5.0.b it is: html5, flash, html:
<javascript>
            var uploadType;
            if (this.browserSupportsHTML5)
            {
               uploadType = this.options.dndUploader;
            }
            else if (this.hasRequiredFlashPlayer)
            {
               uploadType = this.options.flashUploader;
            }
            else
            {
               uploadType = this.options.htmlUploader;
            }
</javascript>
So the change is intentional, I just wasn't aware of it.