Flash programs use .8ck and assembly programs use .8xp, personally I use the extension .8cp for assembly programs even though its not official it still loads through TI-Connect. In the file format itself, the program version is set to $0A to prevent the file from being sent to a ti83+/ti84+. 84 has $01, 83 probably $00. Also instead of using $6DBB for programs, it uses $69EF.