[BUGFIX] EM: uploading extension form must go to iframe

In some browsers the upload form did not go to the iframe for further
processing by JavaScript. Adding a name to the iframe solves the
problem.

Change-Id: If9f6afcf4a80ac178c6c473cc8542ddb4bf7317c
Fixes: #42156
Releases: 6.0
Reviewed-on: http://review.typo3.org/15840
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert