Bug 183919 [Enhancement] add a switch to determine whether to clean viewer image folder when session expires
in current viewer implementation, file in image folder is deleted when its corresponding session expires, could viewer add a switch to allow users to decide whether this deletion operation should be performed.
viewer/org.eclipse.birt.report.viewer/birt/WEB-INF/classes/org/eclipse/birt/report/utility/ParameterAccessor.java
ParameterAccessor.initParameters( config.getServletContext( ) );
		}
	}
	/**
	 * Initial the parameters class. Web.xml is in UTF-8 format. No need to do
	 * encoding convertion.
	 * 
	 * @param context
	 *            Servlet Context
	 */
	public synchronized static void initParameters( ServletContext context )
	{
		if ( isInitContext )
			return;
		
		workingFolder = processWorkingFolder( context, context
				.getInitParameter( INIT_PARAM_WORKING_DIR ) );
		
		documentFolder = processRealPath( context, context
				.getInitParameter( INIT_PARAM_DOCUMENT_FOLDER ),
				IBirtConstants.DEFAULT_DOCUMENT_FOLDER, true );
		
		imageFolder = processRealPath( context, context
				.getInitParameter( ParameterAccessor.INIT_PARAM_IMAGE_DIR ),
				IBirtConstants.DEFAULT_IMAGE_FOLDER, true );
		
		logFolder = processRealPath( context, context
				.getInitParameter( ParameterAccessor.INIT_PARAM_LOG_DIR ),
				IBirtConstants.DEFAULT_LOGS_FOLDER, true );