Can I ask what your server configuration is (apache / php) maybe it is being limited by that.
also this was an interesting read
Also this was posted on the group as well
I wondering if you many need a custom plugin, to create a page header output for the file
I will admit, i thought originally it was for upload, not for download. 
Maybe look at javascript function like this
Also from the grav github , could put you in the right direction
Dangerous extension .js includes .json · Issue #3060 · getgrav/grav · GitHub