Handling file upload MIME validation and size limits in Genesys Cloud Web Messaging integration

refining the guest-side implementation of the Genesys Cloud Web Messaging SDK to ensure strict compliance with our data governance policies regarding file attachments. The primary objective is to prevent users from uploading unsupported file formats or excessively large files before the request reaches the /api/v2/conversations/messaging endpoint, thereby reducing unnecessary payload transmission and potential server-side rejections.

The current implementation use the standard genesys-cloud-messaging-web-sdk version 1.12.4. We have configured the widget with specific attachment constraints as follows:

const widgetConfig = {
 // ... other config
 attachments: {
 allowedTypes: ['image/png', 'image/jpeg', 'application/pdf'],
 maxSize: 5242880 // 5MB in bytes
 }
};

While the UI correctly disables the file input for unsupported types, we are observing a discrepancy when a user attempts to upload a PDF that is slightly larger than the defined maxSize (e.g., 5.1MB). The SDK does not appear to trigger a client-side validation error. Instead, the file is transmitted, and the backend returns a 413 Payload Too Large error. This behavior is problematic because it indicates that the client-side validation logic within the SDK is either not enforcing the maxSize property strictly or is being overridden by a default system limit.

We have also attempted to intercept the upload process using the onAttachmentUpload callback to perform manual validation, but the callback seems to fire after the file has already been processed into a Blob, making it difficult to cancel the operation cleanly without disrupting the user experience.

Is there a documented method to enforce strict file size and MIME type validation entirely on the client side before the SDK initiates the HTTP request? We are looking for a way to display a custom error message to the user immediately upon selection if the file exceeds the 5MB threshold or contains an invalid MIME type, rather than waiting for the API response.

Any insights into the internal validation flow of the Web Messaging SDK regarding attachment constraints would be appreciated.

Check the file size and MIME type in the browser before calling sendFile. The SDK won’t validate this for you, so you need to handle it on the client side.

if (file.size > 5 * 1024 * 1024 || !allowedMimes.includes(file.type)) {
 alert('Invalid file');
 return;
}

MIME sniffing in browsers is unreliable. A user can rename a .exe to .jpg and bypass that check. You need to validate the magic bytes on the backend. If using CXone add a action to inspect the first few bytes of the uploaded file before passing it to the messaging API. It’s safer that way.