Board Thread:Support Requests - Getting Technical/@comment-33878615-20160714182620/@comment-26143371-20160719141807

If you want it back to the default, you can simply delete w:c:ratchet:MediaWiki:wmu-upload, which will reset the message to it's default "Upload".

You can find the names of such messages by appending  (or , if the question mark is already used) to the URL (see Help:System messages).

In this case it was a little more complicated because the content is loaded via AJAX, so it was necessary to find the URL of the popup first (done with the network tab of the browsers developer tools).

There you can find the name of the system-message that you have to edit (the one I linked above).