This rebases and integrates #4209, removing the sniffer task (turns out it wasn't a great idea), and adds a `--sniff-mime-types` command line flag to enable sniffing for file:// and http:// resources. Tested against a random picture file on my harddrive. The actual MIME sniffing implementation can be extracted into a separate library separately.
Source-Repo: https://github.com/servo/servo
Source-Revision: c7e210f24c97be1057a652b3644332e7043bfeac
