FileAttachment.content_bytes does not return b64-encoded content.

Describe the bug

On attempt to b64decode the content_bytes of a file attachment the following error occurs:
binascii.Error: Only base64 data is allowed
The same test provided below passes with msgraph-sdk <= 1.58.0
The code comments still suggest b64-encoded content:

# The base64-encoded contents of the file.
content_bytes: Optional[bytes] = None

Expected behavior

FileAttachment.content_bytes returns b64-encoded data.

How to reproduce

from base64 import b64decode
from typing import cast

from azure.core.credentials import TokenCredential
from azure.identity import ClientSecretCredential
from msgraph import GraphServiceClient
from msgraph.generated.models.attachment import Attachment
from msgraph.generated.models.file_attachment import FileAttachment
from settings import settings

credentials = cast(
    TokenCredential,
    ClientSecretCredential(
        tenant_id=settings().microsoft_graph_tenant_id,
        client_id=settings().microsoft_graph_client_id,
        client_secret=settings().microsoft_graph_client_secret,
    ),
)
scopes = ["https://graph.microsoft.com/.default"]
client = GraphServiceClient(credentials, scopes)


async def get_mail_attachments(
    mailbox: str, mail_id: str
) -> list[Attachment]:
    response = (
        await client.users.by_user_id(mailbox)
        .messages.by_message_id(mail_id)
        .attachments.get()
    )
    attachments: list[Attachment] = response.value or []
    return attachments


async def test_attachments():
    attachments = await get_mail_attachments("my-mailbox", "my-mail-id")
    assert len(attachments) >= 1
    first_attachment = attachments[0]
    assert isinstance(first_attachment, FileAttachment)
    b64decode(first_attachment.content_bytes, validate=True)

SDK Version

1.60.0

Latest version known to work for scenario above?

1.58.0

Known Workarounds

No response

Debug output

</details>


### Configuration

_No response_

### Other information

_No response_

从 FileAttachment 模型的 content_bytes 字段以及复现中的附件检索调用开始。比较 SDK 版本 1.58.0 和 1.60.0 之间的行为，然后对检索到的附件运行提供的 b64decode 验证。完成的标准是：按照描述，content_bytes 可以在 validate=True 的情况下传递给 b64decode。