do you want to do it via code or would you prefer formulaes?? possible with both.
if you make the assumption that the word you want is split by '_' start and end you can try the below ...
=MID(A1,(FIND("_",A1,1)+1),(FIND("_",A1,(FIND("_",A1,1)+1))-(FIND("_",A1,1)+1)))
name of file in A1
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