Hi,
As Javier said, you can use file name mask for deciding which file to be picked. This file name mask is usual regular expression. You can find the sample from below thread.
SFTP sender adapter filename in regular expression
File Exculsions in SFTP - regular expressions
Sample regular expression syntax can be found on the below link:
You might need regular expression like below:
^[A-Za-z0-9]{4}[0-9]{1}[A-Za-z0-9]{10}[0-9]{4}_[0-9]{2}$