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
{
			filePath = workingFolder + File.separator + filePath;
		}
		return filePath;
	}
	/**
	 * Get report document name. If passed file path is null, get document file
	 * from request. If isCreated is true, try to create the document file when
	 * file path is null.
	 * 
	 * @param request
	 * @param filePath
	 * @param isCreated
	 * @return
	 */
	public static String getReportDocument( HttpServletRequest request,
			String filePath, boolean isCreated )
	{
		if ( filePath == null )
			filePath = DataUtil.trimString( getParameter( request,
					PARAM_REPORT_DOCUMENT ) );
		
		if ( filePath.length( ) <= 0 && !isCreated )
			return null;
		if ( filePath.length( ) <= 0 )
		{
			filePath = generateDocumentFromReport( request );
			filePath = createDocumentPath( filePath, request );
		}
		else