These are the new "helper" blocks, they show up whereever you can add an asset to something - e.g. image component as well, but not in the file component 
This is a new feature that no other builder has.
If you had any image files in the Media Folder, their names would appear here in case you wanted to add a background image to a Button. No one thought to filter the list of files by type.
There is a filtering function in helpers blocks. I tested in the extension, it works. It is possible that it has not been added in the components.
That's correct. We were concerned there might be corner cases where a particular format (image, audio, etc.) was supported by the user device and we'd accidentally break the app by kicking out what appeared to be an invalid asset based on the chose supported extensions.
Exactly. People sometimes have such ideas that they will break even the most thoughtful code ...
I stand corrected.
Thanks a lot to all of you.
Now I understand the purpose, and how it works what the release notes
just mentioned.