Question: Is there a way to write a file to local hard disk

I am running autotests with emrun and everything works fine.
On test errors I save screenshots to emscripten IndexedDB.
I know that it is not directly possible to write it as well on the local hard disk but I wonder if there are some workarounds.
I am thinking of "writing" the file to stdout and then rebuilding it from the log file. I guess that should make the trick but do you have other advices ?
Thanks!