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.
All the best,
the Telerik team
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