http://chriskief.com/2014/07/13/setting-s3-cache-metadata/

s3.py

from boto.s3.connection import S3Connection
    
    connection = S3Connection('aws access key', 'aws secret key')
    
    buckets = connection.get_all_buckets()
    
    for bucket in buckets:
        for key in bucket.list():
            print('%s' % key)	
    
            if key.name.endswith('.jpg'):
                contentType = 'image/jpeg'
            elif key.name.endswith('.png'):
                contentType = 'image/png'
            else:
                continue
    
            key.metadata.update({
                'Content-Type': contentType,
                'Cache-Control': 'max-age=864000'
            })
            key.copy(
                key.bucket.name,
                key.name,
                key.metadata,
                preserve_acl=True
            )