n8n Fails to Recognize Downloaded PNG as Binary When Writing File to Disk

Below is a summary of what’s happening and how you can resolve it:

What’s the issue?

The error
  "This operation expects the nodes input to be binary"
indicates that when the Write File node (Read/Write Files from Disk) runs, it isn’t finding a valid binary property in the incoming data. In other words, while your HTTP Request node does download the PNG file as binary, the next node isn’t receiving it in the way it expects.

Likely causes

1. HTTP Request Node Configuration
    • The HTTP Request node must be set to return a file (i.e. binary) rather than a text response. Check the node’s Response Format option and be sure it’s set to “File”.
    • If the node isn’t returning binary data, then downstream nodes will not recognize the input as binary.

2. Mapping the Binary Property Correctly
    • In the Read/Write Files from Disk node you are using an expression for the Input Binary Field (named “dataPropertyName” in your workflow snippet).
    • This parameter expects a string that names the binary property (for example, the default property is often called “data”).
    • Using an expression like ={{ $json.data }} might incorrectly evaluate to the content of the binary data rather than the name of the field.

How to fix it

• Ensure HTTP Request Returns Binary Data
   • In your HTTP Request node, set the response format to “File”. This will make sure the file is returned as binary and stored under the node’s binary property (typically named “data”).

• Reference the Binary Property Name Correctly
   • Instead of using an expression that returns the data content, simply provide the binary field’s name as a string. For example, if the binary data is stored under the property “data” you should set the Input Binary Field (or “dataPropertyName”) to:
    data

Here is an example of a corrected configuration for the Read/Write Files from Disk node:

JSONCopied
{
  "parameters": {
    "operation": "write",
    "fileName": "={{ $('Wordpress1').item.json.slug }}.png",
    "dataPropertyName": "data", // Use the field name here, not the binary content
    "options": {}
  },
  "name": "Read/Write Files from Disk",
  "type": "n8n-nodes-base.readWriteFile",
  "typeVersion": 1,
  "position": [1660, -300]
}

Recap

• Verify that the HTTP Request node is configured to return a file (binary) by setting its Response Format to “File”.
• In the Read/Write Files from Disk node, make sure you reference the binary field by its key (e.g. "data") rather than using an expression that returns its content.

Making these adjustments should resolve the error.