Change the "add new attachment" button

### Description of the bug

The “Add New Attachment” button in the `FileUpload` component, when the `displayMode` property is `simple`, should be displayed as a button rather than as a link-style button.

If it remains in the current link-style format, its font should inherit the font-family (sans-serif) instead of default Arial.

### Steps To Reproduce

Add a FileUpload component with `displayMode` property `simple`.

### Additional Information

_No response_

Locate the FileUpload component and inspect how its simple displayMode renders the “Add New Attachment” control. Reproduce the issue with a simple FileUpload, then verify that the control uses the requested button presentation or inherits the sans-serif font when it remains link-style.

Written by the indexing model from the issue text.