Corrupted file after successfully uploading Excel to task

I am developing an app using R Shiny and trying to upload an Excel as an attachment to a task using the update attachment API call. The file is uploaded successfully via the API, but when downloaded the Excel shows a message that the file is corrupted and cannot be opened.

I have tried various different tweaks to the payload output. I am following exactly what the API reference documentation asks for and the file is uploading successfully.

Here is my existing code that successfully uploads the file. The Payload structure is based on the API documentation:

     url <- <api_url>
     # Read the contents of the Excel file as a raw object
     excel_file_raw <- readBin(excel_file_path, what = "raw", n$size)
     # Encode the Excel file as a base64 string
     file_64_content <- base64enc::dataURI(excel_file_raw,paste0("application/vnd.openxmlformats-     officedocument.spreadsheetml.sheet;name=",file_name), encoding = "base64")
      # Define boundary string
      boundary_string <- "-----boundary"
      # Create payload
     payload <- paste0("--", boundary_string,
               "\r\nContent-Disposition: form-data; name=\"file\"; filename=\"", file_name, "\"",
               "\r\n--", boundary_string,
               "\r\nContent-Disposition: form-data; name=\"parent\"",
               "\r\n\r\n", task_id,
               "\r\n--", boundary_string,
               "\r\nContent-Disposition: form-data; name=\"resource_subtype\"",
               "\r\n--", boundary_string, "--\r\n")

     # Send POST request
     response <- httr::POST(url,
                     body = payload,
                     add_headers('authorization' = paste0("Bearer ", api_token),
                     'content-type' = paste0("multipart/form-data; boundary=", boundary_string),
                     'accept' = 'application/json'),
                     encode = "multipart")

    # Check API response
    content(response, "text")