Support upload more than one file at a time

[matthewma7]

Currently, only one file can be upload at a time. Supporting multiple file upload could speed up the process when a user needs to upload multiple datasets at once.

[matthewma7]

I realize this doesn't work well if we want to support dataset that consists of multiple files, like shapefile. So I would reserve multi-file selection to multi-file upload instead of using multi-file to upload multiple datasets at one time.