Filebeat: send data from a dynamic log + static file version.txt in one event

Hi, we have a dynamic log 'app.log' and a static file version.txt with th version of the app.
We have to send both as a single event with the purpose to have a trace of the errors in function of changing version of the app.
Do you have something in mind how to add the version.txt single value to the 'app.log' stream.
I found that we can use add_filed to append a custom field to the filebeat event, but how to add to this field the value of version.txt file.