I've been thinking a lot about this same subject recently, and I might have some insight into how to access your "files."
Everything that you've learned since this incarnation began forms the framework with which you understand your reality. If you had a memory of something that you experienced before this incarnation, it would appear jumbled and "messy" when interpreted through your current framework. I wonder if that may be why rulers expend so much effort into changing things arbitrarily, because radically changing your framework makes it much more difficult to access older memories.
One specific point of note for me is language. If you wanted to access memories from before (or the early stages of) civilization, to understand how our current world came to be, you would need to eliminate language from your framework. To add to that, I would note that our biological minds function in the quantum world, yet language is firmly grounded in the classical world. I note that our perception of time can change radically in different situations (flow states, dreams) and wonder if that is a result of our brain functioning at a quantum level.