Only allow CSV and JSON files

Covers https://github.com/KSHSK/WAVED/issues/141 - Data upload - verify valid data file extension
Right now, JSON won't work because the resulting data is in a different format than the data for CSV. While the CSV request returns an Array of Objects, the JSON request returns, of course, an Object. Without knowing a key to use on that Object, it can't be in the same format as a CSV request. And even then, there's no guarantee without checking all of the Array entries.