In SMTPAppender the evaluator is used for triggering. In FileAppender, you need to encapsulate the evaluator within a filter, an evaluator filter to be precise. Here is an example:
<property name="USER_HOME" value="c:/temp" />
<appender name="AUDIT_FILE" class="ch.qos.logback.core.FileAppender">
<!-- the filter element -->
<filter class="ch.qos.logback.core.filter.EvaluatorFilter">
<evaluator class="ch.qos.logback.classic.boolex.OnMarkerEvaluator">
<marker>APPEND_SYSLOG</marker>
</evaluator>
<onMismatch>DENY</onMismatch>
<onMatch>NEUTRAL</onMatch>
</filter>
<file>${USER_HOME}/mw_syslog.log</file>
<encoder>
<pattern>%date %level [%thread] %logger{10} [%file:%line] %msg%n</pattern>
</encoder>
</appender>
<root level="debug">
<appender-ref ref="AUDIT_FILE" />
</root>