This is probably not a real bug in the code, but just a problem with the tests. We don't wait for promises to be resolved asynchronously. E.g. in snippets like this:

		transport.checkStatus().fail( tstub );
		...
		assert.ok( tstub.calledWith( 'server-error', { error: {
			code: 'server-error',
			html: mw.message( 'apierror-unknownerror' ).parse()
		} } ) );

This needs to be something like:

		promise = transport.checkStatus().fail( tstub );
		...
		promise.then( null, function () {
			assert.ok( tstub.calledWith( 'server-error', { error: {
				code: 'server-error',
				html: mw.message( 'apierror-unknownerror' ).parse()
			} } ) );
		} );

Change 350871 had a related patch set uploaded (by Matthias Mullie; owner: Matthias Mullie):
[mediawiki/extensions/UploadWizard@master] Update FormDataTransport tests for jQuery 3

https://gerrit.wikimedia.org/r/350871

Change 351019 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/UploadWizard@master] Add our EventLogging schemas to test module's dependencies

https://gerrit.wikimedia.org/r/351019

Change 350871 merged by MarkTraceur:
[mediawiki/extensions/UploadWizard@master] Update FormDataTransport tests for jQuery 3

https://gerrit.wikimedia.org/r/350871

Change 351019 merged by MarkTraceur:
[mediawiki/extensions/UploadWizard@master] Add our EventLogging schemas to test module's dependencies

https://gerrit.wikimedia.org/r/351019