Which fields will be read and written is configured in the
conf/mediameta.php config file, it contains an array
$fields. Each field of the array is an array with the following contents:
fieldname- Where data will be saved (EXIF or IPTC field)
label- key to lookup in the $lang var, if not found printed as is
htmltype- 'text' or 'textarea'
lookups- array additional fields to lookup the data (EXIF or IPTC fields)
Which EXIF and IPTC types are available is listed at detail.php.