[BUGFIX] Improve jumpUrl download of large files

* Prevent output-buffering
* Add Content-Length header

Fixes: #41718
Releases: 6.2, 6.1, 6.0
Change-Id: I8a1a33dc4337a5947bafbad980b73343be637da7
Reviewed-on: https://review.typo3.org/21918
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter