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
for ( int i = 0; i < fileName.length( ); i++ )
		{
			char c = fileName.charAt( i );
			
			if ( c < 0x00 || c >= 0x80 )
			{
				fileName = defaultName;
				break;
			}
		}
		
		String extensionName = getExtensionName( format );
		if ( extensionName != null && extensionName.length( ) > 0 )
		{
			fileName += "." + extensionName; 
		}
		return fileName;
	}
	/**
	 * Reset isInitContext flag
	 */
	public static void reset( )
	{
		isInitContext = false;
	}
	/**
	 * 
	 * Check if OS system is windows
	 * 
	 * @return boolean