Unfortunately, you can't just use an aggregation function on a field like this.
You would need to use a LOAD statement and (most of the time) a GROUP BY clause.
Or you can sort your table descending and pick the first record:
FOR each File IN filelist '../FOLDER/fileNamePrefix_*_fileNameSuffix.ext'
LOAD FileName('$(File)') as Filename,
FileBasename() as FileBasename,
FileExtension() as FileExtension,
FileDir() as FileDir,
//FilePath() as FilePath,
FileSize('$(File)') as FileSize,
FileTime('$(File)') as FileTime
LOAD Filename RESIDENT fileTable ORDER BY Filename desc;
LET mySourceFile = peek('Filename',0,'TMP');
DROP TABLE TMP;