Use the API to get a file uploaded during a response

Hello,
We have a requirement that does two things:
1: Specific survey data gets mapped to Salesforce through response mapping
2: If a file is uploaded to a specific response, we need to get it into Salesforce.
#1 works fine and #2 I can authenticate using the api and oauth and see the response but question is: How do I use the file id to actually get the blob value of the uploaded file?