Use `JSON.parse` to limit exposure

Using `JSON.load` on untrusted input is considered unsafe. While the MIME type
definition files would presumably come from a trusted source, there doesn't seem
to be a need for the "extra" stuff that `JSON.load` does in here, so switching
over to the safer `JSON.parse` API should help to reduce exposure.
Merge pull request #75 from chancancode/use_json_parse

Use `JSON.parse` to limit exposure