add getAdditionalParams(), convert() returns false

inc/inc.ClassConversionServiceBase.php

@@ -49,6 +49,18 @@ abstract class SeedDMS_ConversionServiceBase {
 		return 'Conversion service';
 	}
 
+	public function getAdditionalParams() { /* {{{ */
+		return [];
+	} /* }}} */
+
+	/**
+	 * This method does the conversion
+	 *
+	 * It either returns the content of converted file (if $target is null)
+	 * or writes the converted file into $target and returns true on success
+	 * or false on error.
+	 */
 	public function convert($infile, $target = null, $params = array()) {
+		return false;
 	}
 }