When MIME type cannot be determined, set sensible default

[shaungallagher]

It is possible for getFileMimeType to return false if it is unable to determine the file type. When that happens, an error occurs in React S3 Uploader: Uncaught TypeError: fileType.substr is not a function.

To resolve this issue, the mime-types package recommends that a default be set:

Instead of naively returning the first available type, mime-types simply returns false, so do var type = mime.lookup('unrecognized') || 'application/octet-stream'.