R1:fa687b4ba011

Always ellipsize long filename in file upload dialog

Summary:
Firefox does not ellipsize long filenames after selecting them in the File Upload dialog; Chromium does (for unknown reasons).

Could hardcode the "remaining" width for the `<input>` element itself (approx. 340px) based on calculating the CSS widths and margins of all surrounding elements but that is error-prone if CSS for one of those surrounding elements ever changed.
Thus instead use `max-width: stretch` for the `<input>` element itself. Per…