Async file validation

[cahaseler]

Is your feature request related to a problem? Please describe.
The validator function requires a synchronous function, which means I can't actually read the file in order to do the validation, given that all the browser-friendly ways to do so are async. Not being able to actually look at the contents of the file makes it incredibly hard to do any kind of validation.

Describe the solution you'd like
Accept async javascript.