How can I build a Fat32 image?

[sume520]

I use this tool and want to build a fat32 image, and I use file command to print the info about the image, the terminal print:

test.img: DOS/MBR boot sector, code offset 0xfe+2, OEM-ID "MSDOS5.0", Bytes/sector 4096, FAT 1, root entries 512, sectors 7680 (volumes <=32 MB), Media descriptor 0xf8, sectors/FAT 4, sectors/track 63, heads 255, serial number 0x58973443, unlabeled, FAT (1Y bit by descriptor)

It seems like a fat12 image, and I dont konw how to change the args to build the fat32 image.