amazon server side encryption with post request

I'm trying to use and enforce amazon s3 server side encryption. I followed their documentation and I've created the following bucket policy:

{
   "Version":"2012-10-17",
    "Id":"PutObjPolicy",
    "Statement":[{
        "Sid":"DenyUnEncryptedObjectUploads",
        "Effect":"Deny",
        "Principal":"*",
        "Action":"s3:PutObject",
        "Resource":"arn:aws:s3:::YourBucket/*",
        "Condition":{
           "StringNotEquals":{
              "s3:x-amz-server-side-encryption":"AES256"
           }
        }
     }
  ]
} 

I'm using python boto package, and when I'm adding x-amz-server-side-encryption header its works like a charm.

The problem is that there are several places in the application, that are using a post request from an HTML form to upload files to s3.

I've managed to add the x-amz-server-side-encryption header and the files are uploaded. However, when checking in the amazon backend console I can see that those files are not encrypted.

Does anybody have an idea what I'm doing wrong? I also tried to pass the x-amz-server-side-encryption as a form field but it doesn't help. The interesting part is that when I remove the x-amz-server-side-encryption header, the requests are failing with "access deny" reason.