FileAttachment.content_bytes does not return b64-encoded content.

Start with the FileAttachment model's content_bytes field and the attachment retrieval call in the reproduction. Compare the behavior between SDK versions 1.58.0 and 1.60.0, then run the provided b64decode validation against a retrieved attachment. Done means content_bytes can be passed to b64decode with validate=True as described.

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_