/avatar/upload bug on iOS device

[denkeni]

Describe the issue
"Upload a new avatar" doesn't work on iOS device (it works on macOS).

To reproduce

1. Login on w3.org and go to "My account" https://www.w3.org/users/***/edit/
2. Tap "Upload a new avatar" and select a photo from the album
3. It redirected to the same webpage and showed "There is a problem: Please fix the errors in the form below."

Either on the latest iOS 18 or 26 Safari.

Expected behavior
Successfully uploaded the new avatar.

Additional context
From Safari Web Inspector, the API request POST https://www.w3.org/users/***/avatar/upload/ didn't send the actual file bytes:

--data-raw $'------WebKitFormBoundarya3nD8aahsscfBXut\r\nContent-Disposition: form-data; name="avatar_upload[file]"; filename="C013B479-B0ED-4E26-A9D3-32AF7AA840FE.jpeg"\r\nContent-Type: image/png\r\n\r\n\r\n------WebKitFormBoundarya3nD8aahsscfBXut--\r\n'