要实现此目标，可以使用 AWS Identity and Access Management (IAM) 和 S3 Bucket Policy。下面是一个示例：

首先，创建一个 IAM 策略，将其附加到用户、组或角色上，以允许他们上传特定扩展名的对象，但不允许他们在该桶中创建文件夹。

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:PutObject",
                "s3:GetObject"
            ],
            "Resource": "arn:aws:s3:::bucket-name/*",
            "Condition": {
                "StringLike": {
                    "s3:prefix": [
                        "",
                        "allowed-folder/*"
                    ],
                    "s3:x-amz-meta-filetype": [
                        "allowed-ext1",
                        "allowed-ext2"
                    ]
                }
            }
        },
        {
            "Effect": "Deny",
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::bucket-name/*",
            "Condition": {
                "StringNotEquals": {
                    "s3:x-amz-meta-filetype": [
                        "allowed-ext1",
                        "allowed-ext2"
                    ]
                }
            }
        },
        {
            "Effect": "Deny",
            "Action": [
                "s3:DeleteObject",
                "s3:DeleteObjectVersion",
                "s3:PutObjectAcl",
                "s3:PutObjectVersionAcl",
                "s3:RestoreObject"
            ],
            "Resource": "arn:aws:s3:::bucket-name/*",
            "Condition": {
                "StringNotLike": {
                    "s3:prefix": [
                        "",
                        "allowed-folder/*"
                    ]
                }
            }
        },
        {
            "Effect": "Deny",
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::bucket-name/*",
            "Condition": {
                "Null": {
                    "s3:prefix": true
                }
            }
        }
    ]
}

接下来，创建一个桶策略，限制用户、组或角色不得在 S3 桶中创建文件夹，除非它们有特定的前缀和特定的扩展名。

{
    "Version": "2012-10-17",
    "Statement": [
        {