Add ability to reference files from a custom file migration from other migrations

[rodrigoaguilera]

In this PR we add the feature to have custom file migrations:
#37

I'm pretty sure you can only reference files from the "magic" file migration.
Study how this can be added.

[rodrigoaguilera]

It is something that is doable but I forgot to add a use case for this so prefer to close as there no clear reason why adding the complexity.