add ASSET_MEDIA_TYPES as constant to Asset model
add ASSET_MEDIA_TYPES as constant to Asset model

yangjackascd/roundware-server,Karlamon/roundware-server,probabble/roundware-server,eosrei/roundware-server,jslootbeek/roundware-server,IMAmuseum/roundware-server,Karlamon/roundware-server,probabble/roundware-server,eosrei/roundware-server,IMAmuseum/roundware-server,IMAmuseum/roundware-server,IMAmuseum/roundware-server,probabble/roundware-server,eosrei/roundware-server,jslootbeek/roundware-server,yangjackascd/roundware-server,yangjackascd/roundware-server,Karlamon/roundware-server,jslootbeek/roundware-server,yangjackascd/roundware-server,probabble/roundware-server,jslootbeek/roundware-server,eosrei/roundware-server,Karlamon/roundware-server