{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "s3:GetObject",
            "Resource": "arn:aws:s3:::private-s3-for-interfacing/*"
        }
    ]
}

const AWS = require('aws-sdk');
const S3= new AWS.S3();
exports.handler = async (event, context) => {
    
  let fileName;
  let bucketName;
  let contentType;
  let fileExt;
    
  try {
    bucketName = 'private-s3-for-interfacing';
    fileName = event["queryStringParameters"]['fn']
    contentType = 'application/pdf';
    fileExt = 'pdf';
    
    fileExt = fileName.split('.').pop();
    
    switch (fileExt) {
        case 'pdf': contentType = 'application/pdf'; break;        
        case 'png': contentType = 'image/png'; break;
        case 'gif': contentType = 'image/gif'; break;
        case 'jpeg': case 'jpg': contentType = 'image/jpeg'; break;
        case 'docx': contentType = 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'; break;
        case 'xlsx': contentType = 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'; break;
        default: ;
    }
    
    const data = await S3.getObject({Bucket: bucketName, Key: fileName}).promise();
    
    return {
       headers: {
          'Content-Type': contentType,
          'Content-Disposition': 'attachment; filename=' + fileName,
          'Content-Encoding': 'base64',
          'Access-Control-Allow-Origin': '*',
          'Access-Control-Allow-Headers': 'Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token', 
          'Access-Control-Allow-Methods': 'GET,OPTIONS'
      },
      body: data.Body.toString('base64'),
      isBase64Encoded: true,
      statusCode: 200
    }
  } catch (err) {
    return {
      statusCode: err.statusCode || 400,
      body: err.message || JSON.stringify(err.message)
    }
  }
}