File Extension X01 File Format Secondary index file (Paradox) X01 stands for Secondary index file (Paradox). This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question RPGLE RPGLE multi-file processing with no match fields What is difference between Clustered Index and Non-Cluster Index?