Set mimeType correctly for Wikimedia Commons images

Simply adds the mimeType to images harvested from Wikimedia Commons. Currently this seems to default to image/jpeg for all images - but I have no idea where the default jpeg value is coming from. It is not specified in WikimediaPage.php or the connector, eol_php_code / update_resources / connectors / 71.php.

Hopefully the mimeType set by the code in this pull request will not be overwritten somewhere else in the code by a default "image/jpeg" mime type.