FIX: form template upload type validation (#24148)

When submitting files through the form template upload field, we were having an issue where, although a validation error message was being presented to the user, the upload was still coming through, because `PickFilesButton`'s validation happens **after** the Uppy mixin finished the upload and hit `uploadDone`.

This PR adds a new overridable method to the Uppy mixin and overrides it with the custom validation, which now happens before the file is sent.

Additionally, we're now also using `uploadingOrProcessing` as the source of truth to show the upload/uploading label, which seems more reliable.

spec/system/composer/category_templates_spec.rb

@@ -370,6 +370,7 @@ describe "Composer Form Templates", type: :system do
     expect(find("#dialog-holder .dialog-body p", visible: :all)).to have_content(
       I18n.t("js.pick_files_button.unsupported_file_picked", { types: ".jpg, .png" }),
     )
+    expect(page).to have_no_css(".form-template-field__uploaded-files")
   end
 
   it "creates a post with multiple uploads" do
@@ -401,6 +402,22 @@ describe "Composer Form Templates", type: :system do
     )
   end
 
+  it "overrides uploaded file if allow_multiple false" do
+    topic_title = "Peter Parker's Medication"
+
+    category_page.visit(category_with_upload_template)
+    category_page.new_topic_button.click
+    attach_file "prescription-uploader",
+                "#{Rails.root}/spec/fixtures/images/logo.png",
+                make_visible: true
+    composer.fill_title(topic_title)
+    attach_file "prescription-uploader",
+                "#{Rails.root}/spec/fixtures/images/fake.jpg",
+                make_visible: true
+
+    expect(find(".form-template-field__uploaded-files")).to have_css("li", count: 1)
+  end
+
   it "shows labels and descriptions when a form template is assigned to the category" do
     category_page.visit(category_with_template_6)
     category_page.new_topic_button.click