=SUBSTITUTE(LEFT(CELL("FileName",A1),FIND("]",CELL("FileName",A1))-1),"[","")
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And if you want to play around:
FILENAME: =MID(CELL("FILENAME",A1),FIND("[",CELL("FILENAME",A1))+1,FIND("]",CELL("FILENAME",A1))-FIND("[",CELL("FILENAME",A1))-5)
SHEETNAME: =RIGHT(CELL("FILENAME",A1),LEN(CELL("FILENAME",A1) )-FIND("]",CELL("FILENAME",A1)))