Unable to set file content type in S3

The set_metadata method is really for setting user metadata on S3 objects. Many of the standard HTTP metadata fields have first class attributes to represent them, e.g. content_type. Also, you want to set the metadata before you actually send the object to S3. Something like this should work:

import boto

conn = boto.connect_s3()
bucket = conn.get_bucket('mybucket')  # Assumes bucket already exists
key = bucket.new_key('mykey')
key.content_type = 'text/html'
key.set_contents_from_string(mystring, policy='public-read')

Note that you can set canned ACL policies at the time you write the object to S3 which saves having to make another API call.

For people who need one-liner for this,

import boto3
s3 = boto3.resource('s3')

s3.Bucket('bucketName').put_object(Key='keyName', Body='content or fileData', ContentType='contentType', ACL='check below')

Supported ACL values:

'private'|'public-read'|'public-read-write'|'authenticated-read'|'aws-exec-read'|'bucket-owner-read'|'bucket-owner-full-control'

Arguments supported by put_object can be found here, https://boto3.readthedocs.io/en/latest/reference/services/s3.html#S3.Client.put_object