Fix submit button selector for type-less buttons

Follow-up to #48290.

The `:is(button, input)[type='submit']` selector does not match `button`
elements that omit the `type` attribute in favor relying on its default
value (which is `"submit"`).

Co-authored-by: Javan Makhmali <javan@javan.us>

activestorage/app/javascript/activestorage/ujs.js

@@ -15,9 +15,9 @@ export function start() {
 }
 
 function didClick(event) {
-  const submitButton = event.target.closest(":is(button, input)[type='submit']")
-  if (submitButton && submitButton.form) {
-    submitButtonsByForm.set(submitButton.form, submitButton)
+  const button = event.target.closest("button, input")
+  if (button && button.type === "submit" && button.form) {
+    submitButtonsByForm.set(button.form, button)
   }
 }