extractFilenames: Extract file names from user prompt
Description
This function extracts file names from the user prompt. Current filenames that
are supported by this function are *.txt, *.tsv, *.csv *.xls, *.xlsx,
*.bed, *.bigWig, *.bw and *.bigBed. Other filenames will not be extracted.
If no filenames are found, the function will return NA.