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/context/ViewerAttributeBean.java
if ( lastModifiedOfDesign != -1L && reportDocFile != null
				&& reportDocFile.exists( ) && reportDocFile.isFile( ) )
		{
			if ( lastModifiedOfDesign > reportDocFile.lastModified( )
					|| ParameterAccessor.isOverwrite( request ) )
			{
				reportDocFile.delete( );
			}
		}
	}
	/**
	 * Returns lastModified of report design file. If file doesn't exist, return
	 * -1L;
	 * 
	 * @param request
	 * @return
	 */
	protected long getLastModifiedOfDesign( HttpServletRequest request )
	{
		String designFile = ParameterAccessor.getParameter( request,
				ParameterAccessor.PARAM_REPORT );
		if ( designFile == null )
			return -1L;
		
		File file = new File( this.reportDesignName );
		if ( file != null && file.exists( ) )
		{
			if ( file.isFile( ) )
				return file.lastModified( );
		}