Minimal policy to give to monosnap for s3 upload

monosnap policy aws.json

{
	"Version": "2012-10-17",
	"Statement": [
		{
			"Sid": "AllowGroupToSeeBucketListInTheConsole",
			"Action": [
				"s3:ListAllMyBuckets"
			],
			"Effect": "Allow",
			"Resource": [
				"arn:aws:s3:::*"
			]
		},
		{
			"Sid": "Stmt1410750108000",
			"Effect": "Allow",
			"Action": [
				"s3:GetObjectAcl",
				"s3:GetObject",
				"s3:GetObjectVersionAcl",
				"s3:GetObjectVersion",
				"s3:ListBucket",
				"s3:PutObjectAcl",
				"s3:PutObject",
				"s3:PutObjectVersionAcl"
			],
			"Resource": [
				"arn:aws:s3:::<your-bucket-name>/*"
			]
		}
	]
}

You can remove list all buckets allow statement after you select your desired bucket from monosnap app GUI. Like this you will not give monosnap excessive permissions.