The jira.properties may contain properties to define the lookup strategy for fields.
Usually the default is to lookup a (custom) field by it's name. This can be overwritten, e.g.
jira.attributes.project.resolution=id to overwrite it's default key lookup to id lookup.
public FieldMapper lookup(com.atlassian.jira.rest.client.api.domain.FieldSchema schema)
Returns the FieldMapper, which is registered with schema.