Here are some notes on the usage of the flash filesystem on the LegoMindstorms NXT controller.

File names can be up to 19 characters long, and are case-insensitive. There is no directory hierarchy; all files are kept in a single top-level directory. The on-board interface shows files grouped into various categories, like “Software files”, “Try Me” etc—these groupings are based purely on the filename extension.

 Category   File Contents   Extension 
 Datalog files   text files containing sensor data   .log 
 NXT files   ?   .rpg 
 Software files   executable bytecode programs   .rxe 
 Sound files   sound sample files; melody files   .rso 
 Try Me   executable bytecode programs   .rtm 

Filenames ending with other extensions are not visible to the user through the on-board interface, but they can still be accessed programmatically.