This can happen when Contact Form 7 successfully processes a submission, but the uploaded file isn't being transferred in the format Zoho CRM expects. Your other fields, such as name, email, and phone number, might reach Zoho while the attachment remains missing.

I'd check these three things first:

1. File upload format: Zoho CRM's attachment API expects a multipart/form-data request containing the file. Simply including a local file path or Base64 value in a standard JSON request won't directly upload an attachment through that endpoint.

2. CRM record and permissions: Check that the attachment request uses the correct Zoho module and an existing record ID. Also verify that your OAuth token has the required attachment permissions and that the connected user can upload files.

3. API response logs: Check the actual response rather than relying on the form's success message. Zoho may return errors related to authentication, permissions, an invalid record ID, or the request format.

One practical way to handle the WordPress side of this workflow is Contact Form to Any API. Its Pro version can map Contact Form 7 file fields and convert uploaded files to Base64, while its API logs let you inspect submitted data and the destination's response.

The important part is matching that output to Zoho's requirements. For example, you can send the file data to an endpoint or workflow that converts it into the multipart upload Zoho expects, then attaches it to the appropriate CRM record. If your workflow uses a file URL instead, Zoho also supports attaching a link through its attachment API.

Once the file format, record ID, and permissions are correct, troubleshooting becomes much more straightforward because you can identify whether the issue occurs during form processing, file conversion, or the final upload to Zoho CRM.

Official references:

• Zoho CRM — Upload an Attachment API

• Contact Form to Any API — Documentation and API Logs