Get content from file .txt in folder

Hi. 

I have an external folder with text files, with name format as [somedate]_[Service]_[Transactionid].txt. What i would like to do with that is either, 

make a mockservice that fetch the content of a txt file based on the 'Transactionid' with a script, or somehow else fetch the content from the txt file into my project. 

The request has the transactionid, and can be stores as a property. 

Makes sense?

I do not have PRO license, and not that good with Groovy