We now always trust the file extension to avoid false positive of mime
detection for most simple cases. But we still try to guess the mime type
if the file extension does not match or is missing.
Fixes#564
CSV reader used to accept any file without any kind of check. That made
users incorrectly believe that things were ok, even though there is no
way for CSV reader to read anything else that plain text files.
Fixes#167
Laurent