Add support for uploading multiple files at once.

[malave]

[wrathagom]

The tool I am using in Node-RED would make it very easy to handle multiple file uploads at the same time. So the user can CTRL+click on file1.csv, file2.csv, and file3.csv all at once and upload them all at once. I'll write the flow to be able to handle that.

[wrathagom]

This would require a change to the plugin, but the source would also need updated because the flow doesn't expect to receive more than one file.