GSM file extension is for Sound
GSM 6.10 prI-ETS 300 036 13kbit/s Internet phones
Similar to GSM
#GF - MetaFont Font file
#IB - LocoScript Printer data file
#SC - LocoScript Printer data file
#ST - LocoScript Standard mode printer definition file
$$P - OS/2 Notes file
$$S - OS/2 Spreadsheet
$00 - DOS Pipe file
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