How to Configure Multipart/Form-Data to Send JSON and XML in a Single HTTP Request

Below is an analysis of your issue based on the details provided:

What’s Happening

• With Form Urlencoded, you get an HTTP 415 (Unsupported Media Type). This indicates that the API endpoint doesn’t accept the URLencoded content type.
• With Form Data (set as multipart-form-data), the API returns an HTTP 400 (Bad Request). According to the API documentation you referenced, the expected body must include:
  • One key, file[], marked as a file with content type application/xml.
  • Another key, query, which is a text field that must be sent with the content type application/json.
  • A third key, fileComplement[], also marked as a file.

Possible Causes

1. Mixed Content Types in One Request:
   The API expects a multipart/form-data request that carries both file content and a text key with JSON content. However, n8n’s HTTP Request node might be using a single overall content type (multipart/form-data) and not sending the text parameter with the expected header for application/json (for that key).

2. Incorrect JSON Formatting for the "query" Key:
   The JSON provided in the query parameter must be valid. If the JSON isn’t correctly escaped or formatted, the API may not parse it as expected. For example, ensure that the JSON string is:

   JSONCopied
   {"boxeFile": false}

   and, if using an expression, wrapped correctly to be interpreted as a string. A common solution in n8n is to wrap it in double curly brackets if using expressions.

3. Limitations with the HTTP Request Node for Setting Per-Field Headers:
   The API requires that each field within the multipart/form-data has its own content-type (i.e. file fields vs. text fields with different MIME types). The HTTP Request node in n8n may not allow setting individual content types for each form-data part directly.

Recommendations and Workarounds

• Review JSON Formatting:
  Double-check that the value for the query key exactly matches valid JSON. If using an expression, you might try something like:

  JSCopied
  {{"{\"boxeFile\": false}"}}

  to ensure that n8n sends it as a JSON string.

• Consider Alternative Request Methods:

  • If the API strictly requires different content types per form field, you may want to try using the HTTP Request node with the “Import cURL” option. This way, you can import a cURL command that has been confirmed by the API documentation, which might handle multipart boundaries and per-field headers more precisely.
  • Alternatively, try using a custom request (e.g. via code with a Function node or external script) where you can set the individual part headers.

• Consult the API Documentation:
  Verify if there are any notes regarding sending multipart requests with a mix of file and JSON content. The documentation might provide insights into required boundary formatting or special parameters.

Summary

The root issue seems to be that the API expects a multipart/form-data payload with specific per-field MIME types (one for files and another for a JSON text field). Using URL-encoded data results in HTTP 415, and using default form-data without per-field adjustments results in HTTP 400. You will need to ensure:

• The query field’s JSON is correctly formatted and sent with the application/json header if possible.
• The file fields (file[] and fileComplement[]) use the correct content type (application/xml for file[]).

If n8n’s HTTP Request node does not allow per-field control over the content-type, you might need to use the cURL import feature or a different approach to ensure the request meets the API’s expectations.

Feel free to ask if you need further clarification on any of these points.