FILES gives an error when checking projects with certain names

FILES usually never causes errors, even if the specified project doesn't exist However, certain valid project names will cause errors with FILES. These are:

• Any name containing @ (this can only be created through hacking)
• DOS reserved device names (CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, LPT1, LPT2, LPT3, LPT4) (even though they can be created without hacking)

If you're designing a file viewer, be sure to check for these names, or it will cause your program to give an error if someone has made a project named "NUL" or something

Interesting. Do you think that is caused by SmileBASIC's own code, or the 3DS's interface for the SD card File System?

it's smilebasic and because they just used whatever windows-style filesystem library they had.