Thanks a lot for all the valuable suggestions, please find below the final working script for reference:
dt= Multiple File Import(
<<Set Folder(
"C:\Users\my_files\"
),
<<Set Name Filter( "*.*;" ),
<<Set Name Enable( 0 ),
<<Set Size Filter( {37111019, 38896395} ),
<<Set Size Enable( 0 ),
<<Set Date Filter( {3689368844.046, 3689369157.819} ),
<<Set Date Enable( 0 ),
<<Set Add File Name Column( 1 ),
<<Set Add File Size Column( 0 ),
<<Set Add File Date Column( 0 ),
<<Set Import Mode( "CSVData" ),
<<Set Charset( "Best Guess" ),
<<Set Stack Mode( "Stack Similar" ),
<<Set CSV Has Headers( 1 ),
<<Set CSV Allow Numeric( 1 ),
<<Set CSV First Header Line( 1 ),
<<Set CSV Number Of Header Lines( 1 ),
<<Set CSV First Data Line( 2 ),
<<Set CSV EOF Comma( 1 ),
<<Set CSV EOF Tab( 0 ),
<<Set CSV EOF Space( 0 ),
<<Set CSV EOF Spaces( 0 ),
<<Set CSV EOF Other( "" ),
<<Set CSV EOL CRLF( 1 ),
<<Set CSV EOL CR( 1 ),
<<Set CSV EOL LF( 1 ),
<<Set CSV EOL Semicolon( 0 ),
<<Set CSV EOL Other( "" ),
<<Set CSV Quote( "\!"" ),
<<Set CSV Escape( "" )
) << Import Data;
dtx = dt[1];
sel = dtx << Get Rows Where ( dtx:A <= 20);
dtx << delete rows(sel);
dtx<<
Summary(
Group( dtx:File Name ),
Mean( dtx:A ),
Freq( "None" ),
Weight( "None" )
)