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File uploads can fail for the same reasons as any submission: invalid form ID, storage misconfiguration, or network issues. The Formbase submit endpoint responds with standard HTTP status codes so you can handle errors in your UI. Here is a simple client-side pattern you can reuse. 
const response = await fetch('https://formbase.dev/s/YOUR_FORM_ID', {
  method: 'POST',
  body: new FormData(formElement),
  redirect: 'manual',
});

if (response.status === 303 || response.ok) {
  // Success
} else if (response.status === 404) {
  // Form not found
} else {
  // General failure (500 or other)
}

​
Common error causes

Formbase throws an error if storage credentials are missing when a file is uploaded. Ensure all STORAGE_* environment variables are set.
A 404 response means the form ID does not exist or is not available in this Formbase instance.
Handle timeouts or offline conditions by showing a retry button and preserving form state when possible.
Browser submissions return a 303 redirect on success. Treat 303 as a successful upload.