How to import a Text File from feeds CSV and map it to a File upload field with multiple values?
It looks like you're on the right track from:
https://www.drupal.org/node/1116130#comment-6827626

I believe you to set it to use an array of values (that's what explode returns). The file type itself, image vs text file, is largely irrelevant as you simply want it to work with a multiple item field.