Document Attachment in Sales Order in Business Central — "Read called with an open stream" Error

I’m trying to create a document attachment in Business Central using this API:

🔗 POST /documentAttachments

However, I always get the following error:

Read called with an open stream or textreader. Please close any open streams or text readers before calling Read. CorrelationId: bee212fd-2651-45e1-b57c-84926816a7ea

I’ve already tried several variations of base64 encoding, but no luck so far. Here's the current version of my function (written in vue3, using Axios):

const attachFileToBCModule = async (payload) => {
  const { module, recordId, file, config } = payload;

  const attachmentEndpoint = replaceUrlPlaceholders(attachmentEndpointFormat, config);

  try {
    const token = await acquireToken('businesscentral');

    // Process file completely before making API call
    const fileContent = await file.arrayBuffer();
    const uint8Array = new Uint8Array(fileContent);
    const binaryString = Array.from(uint8Array, byte => String.fromCharCode(byte)).join('');
    const base64Content = btoa(binaryString);

    const fileName = file.name;
    const fileSize = file.size;

    const uploadUrl2 = `${attachmentEndpoint}/documentAttachments`;

    const uploadResponse2 = await axios.post(
      uploadUrl2,
      {
        fileName: fileName,
        byteSize: fileSize,
        attachmentContent: base64Content,
        parentId: recordId,
        parentType: 'Sales Order',
      },
      {
        headers: {
          Authorization: `Bearer ${token}`,
          'Content-Type': 'application/json',
        },
      }
    );

    return {
      status: 201,
      message: `Attachment uploaded to ${module} successfully.`,
      data: { upload2: uploadResponse2.data },
    };

  } catch (err) {
    console.error('Error uploading attachment:', err?.response?.data || err.message);
    return {
      status: err.response?.status || 400,
      error: err.response?.data?.error?.message || err.response?.data || 'Attachment upload failed',
    };
  }
};

For reference, I also tried the other endpoint:

🔗 POST /attachments

That one works fine—but it uploads the file to Incoming Documents, not the Attachments panel in the Sales Order. From what I understand, the correct endpoint is /documentAttachments.

I found a solution!

To resolve the issue, I created a custom API for Document Attachment in Business Central.

Here’s what I did:

1. Go to the Web Services module in Business Central.

2. Add a new Web Service:

   • Object Type: Page

   • Object ID: 30080 (this is the ID of the Document Attachment page)

   • Service Name: DocumentAttachments (or any name you prefer for the endpoint)

After publishing this, you can now consume the API using a POST and PATCH request. Here’s the revised code I used:

const attachV2DocumentAttachments = async (payload) => {
    const { module, createdOrder, documentFile, config } = payload
    
    const v2DocumentAttachmentsEndpoint = replaceUrlPlaceholders(v2DocumentAttachmentsEndpointFormat, config)
    const attachmentUrl = `${v2DocumentAttachmentsEndpoint}`
    
    try {
      const token = await acquireToken('businesscentral')
      
      // Step 1: Create attachment record with metadata only
      const attachmentRecord = {
        parentId: createdOrder.id,
        fileName: documentFile.name,
        parentType: 'Sales Order'
      }
      
      console.log('Creating attachment record:', attachmentRecord)
      
      const createResponse = await axios.post(
        attachmentUrl,
        attachmentRecord,
        {
          headers: {
            Authorization: `Bearer ${token}`,
            'Content-Type': 'application/json',
          },
        }
      )

      console.log('createResponse', createResponse)
      
      const attachmentId = createResponse.data.id
      console.log('Attachment record created with ID:', attachmentId)
      
      // Step 2: Upload the actual file content
      const v2DocumentAttachmentsEndpoint = replaceUrlPlaceholders(v2DocumentAttachmentsEndpointFormat, config)
      const v2DocumentAttachmentsURL = `${v2DocumentAttachmentsEndpoint}(${attachmentId})/attachmentContent`
      
      const uploadResponse = await axios.patch(
        v2DocumentAttachmentsURL,
        documentFile,
        {
          headers: {
            Authorization: `Bearer ${token}`,
            'Content-Type': documentFile.type || 'application/octet-stream',
            'If-Match': '*'
          },
        }
      )

      return {
        status: 201,
        message: `Attachment uploaded to ${module} successfully.`,
        data: {
          attachment: createResponse.data,
          upload: uploadResponse.data
        },
      }
        
    } catch (err) {
      console.error('Error uploading attachment:', err?.response?.data || err.message)
      return {
        status: err.response?.status || 400,
        error: err.response?.data?.error?.message || err.response?.data || 'Attachment upload failed',
      }
    }
  }