When you try to send mms, does it give you a blue outline around the picture? if you have it instead of an error complaining about missing s3 and the mms came to your cell, most likely it worked, and the PBX will create a folder structure in your S3 bucket that will look like this:
bucketname-->
Inside today's date folder, you should see your picture.
When I send a mms with a image it has a blue box around it and a error message, the cell receives no message. If I send with no image the message is delivered. No file is created in bucket.