How to access user uploaded files in Copilot Studio?

In Microsoft Copilot Studio, how can I extract and store the text content of a Word or PDF file uploaded by an end user via the chat UI (using the attachment icon) into a variable for further processing?

For example, I want to:

1. Extract the file's text and store it in a variable.
2. Use this text in a workflow to send it to a private GPT instance with a custom prompt to generate a structured summary.

This question is not related to the "Add Content" feature for knowledge file uploads but focuses on handling files uploaded during live chat interactions.

Any guidance or documentation would be greatly appreciated!