Support asynchronous input-filter

[jacobg]

Currently this component relies on the @input-filter event handler synchronously calling prevent() to reject a file from being dropped on the component. The limitation of this approach is that this filter handler will not be able to asynchronously read the file to do advanced filtering such as checking for prohibited file magic numbers:
https://developer.mozilla.org/en-US/docs/Web/API/Blob/arrayBuffer

Please enhance this component to support asynchronous input filters that return a promise.

Thanks!