How to send user uploaded file to custom Rest API endpoint?

Question

How to send user uploaded file to custom Rest API endpoint?

Nguyen Nhu Phuc 20

Hi technical support team.

We have an existing document intelligence platform to serve out clients. Usually, a user would log in to our platform through out website, upload files to get business insights back.

We are trying to simplify/unify the way our clients use the app, by creating a Microsoft 365 Copilot agent and let the user interact with our platform through this agent. A user would select a starter prompt for a specific task, upload a file and then the agent will send an HTTP request containing the starter prompt and the file to our backend. The main idea is to make use of entraID to skip authentication, our client can just chat with our client through Teams to do what they have been doing.

I've gone through the Copilot bootcamp and several official documentations, however, I couldn't find anything related to this task. The closest I could get to was something like file metadata like this by using Power Fx variable Activity.Attachments in Copilot Studio. It seems that base64 encoded file content is no longer available through this variable.

[{"[
  {
    "Content": {
      "$kind": "FileDataValue",
      "value": {
        "$kind": "ConversationFileReference",
        "value": "9ce2ec8c-4924-40ad-aba2-b849276ca048"
      }
    },
    "ContentType": "application/pdf",
    "Name": "user upload file.pdf",
    "Value": {
      "$kind": "FileDataValue",
      "value": {
        "$kind": "ConversationFileReference",
        "value": "9ce2ec8c-4924-40ad-aba2-b849276ca048"
      }
    }
  }
]

I also tried using declarative agent but it doesn't seem to support forwarding file using HTTP request.

I am trying to debug a custom engine agent using Microsoft 365 Agents Playground but it doesn't allow file upload.

I hope you guys can give me some insights to overcome this issue. Any suggestion or link to documentation is appreciated.

Thank you very much.

Sayali-MSFT 3,911 Reputation points Microsoft External Staff Moderator

2025-06-20T05:37:09.9166667+00:00

@Nguyen Nhu Phuc,Thank you for bringing this issue to our attention. We will look into it and get back to you shortly.

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Sayali-MSFT 3,911 Reputation points Microsoft External Staff Moderator

2025-06-20T05:37:09.9166667+00:00

@Nguyen Nhu Phuc,Thank you for bringing this issue to our attention. We will look into it and get back to you shortly.

Answer 1

Sayali-MSFT 3,911 Microsoft External Staff Moderator

Hello @Nguyen Nhu Phuc,You only get a ConversationFileReference (an opaque file reference/token) in Activity.Attachments. You do NOT get the file content or a direct download URL. There is no built-in way to forward the actual file content to your backend via HTTP from a declarative agent.
Declarative agents and Copilot Studio are currently not suitable for this use case.

If you need to process file content, Build a custom Teams bot using the Bot Framework SDK (C# or Python). This bot can receive file uploads, download the file using the contentUrl, and send it to your backend.
Reference Document-https://learn.microsoft.com/en-us/microsoftteams/platform/bots/how-to/bots-filesv4?tabs=csharp1%2Ccsharp

import aiohttp

async def download_teams_file(content_url, token):
    headers = {"Authorization": f"Bearer {token}"}
    async with aiohttp.ClientSession() as session:
        async with session.get(content_url, headers=headers) as resp:
            if resp.status == 200:
                return await resp.read()
            else:
                raise Exception(f"Failed to download file: {resp.status}")

Nguyen Nhu Phuc 20 Reputation points

2025-06-23T13:36:14.3033333+00:00
Thank you for your response. If it is a Teams Bot, can it be used in Microsoft 365 Copilot? Our main target is MS365 Copilot.

I am trying the custom engine agent approach, the Typescript SDK seems to provide a way to download Attachment through:

/** * Gets the content of an attachment. * * @param attachmentId The ID of the attachment * @param viewId The view to get * @returns A promise that resolves to a readable stream of the attachment content */ getAttachment(attachmentId: string, viewId: string): Promise<NodeJS.ReadableStream>;

Where attachment metadata is accessible through:

context.activity.attachments

However, when I debug it using Agent Toolkit, the attachments are not there (undefined).

I already modify the manifest.json to set supportsFiles = true

Am I understanding the SDK incorrectly, or the attachments are not included during runtime?

Thank you!
Sayali-MSFT 3,911 Reputation points Microsoft External Staff Moderator

2025-06-25T11:47:03.54+00:00
Hello @Nguyen Nhu Phuc,

Can a Teams Bot be used in Microsoft 365 Copilot?

Not all Teams bots are automatically available in Copilot.

To be used in Microsoft 365 Copilot, your bot must be registered as a Copilot plugin/agent and meet Copilot extensibility requirements (see Copilot extensibility docs).

If you build your bot using the Microsoft 365 Agents Toolkit, it can be surfaced in Copilot experiences.

Attachments in Copilot Custom Agents

In Teams, when a user uploads a file, the bot receives it as an attachment in context.activity.attachments.

In Copilot custom agents (using the Agent Toolkit), as of June 2025, file attachments are not always passed through to the agent—especially in the Playground or some Copilot chat surfaces.

This is a known limitation: the Copilot runtime may not yet support file upload or attachment flows for custom agents.

Even if you set "supportsFiles": true in your manifest, the runtime must support file upload for attachments to appear.

TypeScript SDK: getAttachment

The method you referenced (getAttachment(attachmentId, viewId)) is correct for downloading file content if the attachment is present in context.activity.attachments.

If attachments is undefined or empty, it means the runtime did not pass any files to your agent. You are using the SDK correctly, but the Copilot runtime does not currently pass file attachments to custom agents, even with supportsFiles: true. This is a platform limitation, not a code issue.
Nguyen Nhu Phuc 20 Reputation points

2025-06-26T02:55:40.74+00:00
Thank you for your response. I would like to connect all the dots you gave above with one more question.

Can we work with the attachments if we:

Build a Teams Bot using Bot SDK

Register it as a Copilot Plugin/Agent and meet extensibility requirement

Our users use the Agent in Copilot
Sayali-MSFT 3,911 Reputation points Microsoft External Staff Moderator

2025-06-27T09:05:20.7833333+00:00

Hello @Nguyen Nhu Phuc, you cannot reliably work with attachments in Copilot agents, even if you build your bot using the Bot Framework SDK and register it as a Copilot plugin/agent.

Teams Bot in Teams: Yes, you can access attachments via context.activity.attachments and download file content.

Copilot Plugin/Agent: Even if your bot is registered as a Copilot agent and has "supportsFiles": true in the manifest, Copilot does not currently pass file attachments to your agent. The attachments array will be empty or undefined when invoked from Copilot.

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How to send user uploaded file to custom Rest API endpoint?

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