This can be done by setting the bucket policy for S3. If not, you will have to set the permission for every object that you upload. Following is the policy which make all the objects public automatically:
{
"Version": "2008-10-17", "Id": "http better policy",
"Statement": [
{
"Sid": "readonly policy",
"Effect": "Allow",
"Principal": "*",
"Action": "s3:GetObject",
"Resource": "arn:aws:s3:::mybucket/sub/dirs/are/supported/*"
}
]
}