Attach or select files from filepicker fields

Many QA tests and moodle features requires files to be attached or repository files to be selected. We need a step to interact with filepicker fields and also the I fill the moodle form with: step should support it.

Most of the arguments will depend on the selected repository so a flexible argument should be used, I propose something like I attach "REPOSITORY_DEPENDANT" from "REPOSITORY_NAME" repository to "FIELD_NAME" field where REPOSITORY_DEPENDANT varies, being the path to a file using "Upload a file" repo or a file name in "Recent files"

This issue could be split in subtasks for the different repositories, so we can begin with the basic framework to support multiple repositories and the "Upload a file" repo, to allow test writers to write features that depends on the filepicker and progresively add more repositories.