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 its limited support (see https://caniuse.com/?search=max-width), set also -moz-available (https://bugzilla.mozilla.org/show_bug.cgi?id=1495868) and -webkit-fill-available (see https://bugs.chromium.org/p/chromium/issues/detail?id=611857) aliases.