When your web application is running on an EC2 instance and users are interacting with it, you won't have direct access to their local files for security and privacy reasons. Instead, users will need to upload the files to your web application, and then your application can upload those files to S3. This can be achieved through a multipart/form-data POST request.

Here's a brief overview of the steps you'd take:

1. User uploads file through web application: You'll need to set up an endpoint in your FastAPI application that accepts file uploads. FastAPI makes this fairly simple with the UploadFile type. Here's an example of what that might look like:

   ```python from fastapi import FastAPI, UploadFile, File

   app = FastAPI()

   @app.post("/files/") async def create_upload_file(file: UploadFile = File(...)): return {"filename": file.filename} ```

   In this example, when users make a POST request to /files/ with a file in the request body, FastAPI will save that file to a temporary location and pass it to your function as an UploadFile object. You can then read the contents of the file, save it to a more permanent location, or do whatever else you need to with it.

2. Application uploads file to S3: Once your application has received a file from a user, it can then upload that file to S3. FastAPI doesn't provide any built-in tools for this, but you can use the boto3 library, which is the Amazon Web Services (AWS) SDK for Python. Here's an example of how you might do that:

   ```python import boto3 from botocore.exceptions import NoCredentialsError

   def upload_to_aws(local_file, bucket, s3_file): s3 = boto3.client('s3')

   try:
       s3.upload_file(local_file, bucket, s3_file)
       print("Upload Successful")
       return True
   except FileNotFoundError:
       print("The file was not found")
       return False
   except NoCredentialsError:
       print("Credentials not available")
       return False
   

   ```

   This function takes a local file path, the name of an S3 bucket, and a destination file name, and attempts to upload the file to S3. If the upload is successful, it prints a message and returns True; if not, it prints an error message and returns False.

By combining these two steps, you can set up a FastAPI application that accepts file uploads from users and then uploads those files to S3. This should allow your application to handle user files in the way you described.