asyncAddFiles fixes

[ilessiivi]

• Cleaned up the formatting a bit, removed unused variable
• Adds an extra filtering between file-added and files-added hooks to unify the behavior between sync and async code

[drzraf]

The cleanup part is minor and the change from this.hook => await this.aHook will soon be changed again to await this.hook.
I suggest to discard it so that the functional change can happen in the bigger async-only PR where it makes more sense.
(And the small cleanup about variable can be forgotten for now)
I think it'll be simpler to manager your multiple PR (good job btw) and a more readable git history.

[ilessiivi]

I'll close this PR, the changes will be in #354.