file attachment by DnD

Hi there,

Thanks for doing outline, it's is simple to run and really effective.
I've upgraded the docker version to 0.63.0-1 to try file attachment, it's working fine, it would be better if that could be done by just doing a drag and drop from file manager application.
I've also notice this is possible to only attach one file at the time, could it be possible to attach several files at once.

Best