_h5ai/client/js/scripts.js の下記の行を変更
return jQuery.isFunction(u) && (u = u(t)), s += o, s += i, s += a, s += 'Content-Disposition: form-data; name="' + (u || "") + '"', s += '; filename="' + t + '"', s += a, s += "Content-Type: " + r, s += a, s += a, s += n, s += a, s += o, s += i, s += o, s += a
'; filename="' + t
↓
'; filename="' + encodeURIComponent(t)
_h5ai/server/php/inc/Api.php
$dest = $upload_dir . "/" . utf8_encode($userfile["name"]);
↓
dest = $upload_dir . "/" . urldecode($userfile["name"]);