Rasa Open Source itself cannot process excel files, only text messages. If you want your bot to do anything else than receiving and sending text messages, you need to run an action server and write a Custom action that does the task.
So in your case, you could have the button click send an intent /upload_faq to the bot and have this intent trigger your custom action action_upload_faq. See our documentation on the MappingPolicy for how to do this.
All of the following tasks need to be done by your implementation of the action:
let the user upload the file
read the file and extract the information
make the information available to Rasa, e.g. by writing it into slots or by storing it in a database and having some more custom actions with which Rasa accesses this database if needed
If you want to implement the action in Python, check out our Rasa SDK. It provides a framework in which you can implement custom actions and it can start the action server for you.
Let me know if you have any further questions about this approach.