Hi @anssiko 
The use case behind the requirement is to allow apps to access urls, which is not limited to file:// but also include others like http/https. That may load a js file or some other resources via XHR from a website. If you want to know the exact requirement, @gaochun, who is supporting the user asked for this, may help provide more details. So file picker could not meet the case.
Exposing the flag of allow-universal-access-from-file-urls to app developers is much straightforward way to meet this kind of requirement. Of cause for us/web runtime, we should always advise not to open this flag because of security concern. 
So we need to enable the flag in blink on app scheme by recognizing it as a local one.