Fix mimetype detection in getmedia
Fix mimetype detection in getmedia

In some server, the magic database does not contain pattern for pdf file
so it returns application/octet-stream.

nboisteault/lizmap-web-client,rldhont/lizmap-web-client,3liz/lizmap-web-client,3liz/lizmap-web-client,3liz/lizmap-web-client,nboisteault/lizmap-web-client,rldhont/lizmap-web-client,rldhont/lizmap-web-client,laurentj/lizmap-web-client,laurentj/lizmap-web-client,3liz/lizmap-web-client,laurentj/lizmap-web-client,rldhont/lizmap-web-client,laurentj/lizmap-web-client,nboisteault/lizmap-web-client,nboisteault/lizmap-web-client