It’s not possible to Open and modify attached files directly instead of downloading them yet.
And this is what our user docs say about the location of these files:
Files are stored inside
flatfs dirin encrypted fragments, so they can’t be accessed outside of Anytype.
So when you open an object with an image, it downloads. When you press play on video & audio, it begins to download. After that, this file will be stored in the application cache.