HOWTO: Set content-type for images before upload to Amazon S3 storage

// Add Content-Type value for uploaded images
uploader.metadata().addCustomField('Content-Type', 'image/jpeg');

private static string ConstructPolicy(string bucket, DateTime expirationDate, string acl, string key)
	{
		// See about policy construction 
		// http://docs.amazonwebservices.com/AmazonS3/2006-03-01/dev/HTTPPOSTForms.html#HTTPPOSTConstructPolicy	

		string policy = string.Format(@"
			{{ 'expiration': '{0}.000Z',
				'conditions': [
					{{ 'acl': '{1}' }},
					{{ 'bucket': '{2}' }},
					{{ 'success_action_status': '200' }},
					[ 'starts-with', '$key', '' ],
					[ 'starts-with', '$Content-Type', '' ],
				]
			}}", expirationDate.ToString("s"), acl, bucket);

		// Encode the policy using UTF-8.
		byte[] pb = System.Text.Encoding.UTF8.GetBytes(policy.ToString());

		// Encode those UTF-8 bytes using Base64 and return.
		return Convert.ToBase64String(pb);
	}