This is the PITS item for this requestm, you can vote to increase its priority. Implementation is not scheduled for Sitefinity 5.0 release. I can`t give a time frame when this will be implemented.
As a different type of solution for this is to use this sample module that implements search in the contents of .txt files. I have attached a video demonstrating how to add the module to sitefinity project. When creating new search index there will be new option Documents that will search for the contents of .txt files.