Limit previous solution for non-push mode UI (#11629)

* add test case

* Limit the workaround for non-push mode

Fixes #11616

server/src/main/java/com/vaadin/server/communication/FileUploadHandler.java

 
 package com.vaadin.server.communication;
 
-import static java.nio.charset.StandardCharsets.UTF_8;
-
 import java.io.BufferedWriter;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import com.vaadin.ui.UI;
 import com.vaadin.ui.Upload.FailedEvent;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 /**
  * Handles a file upload request submitted via an Upload component.
  *
             } finally {
                 session.unlock();
             }
-            return true;
+            boolean pushEnabled = UI.getCurrent().getPushConfiguration()
+                    .getPushMode().isEnabled();
+            if (!pushEnabled) {
+                return true;
+            }
+
             // Note, we are not throwing interrupted exception forward as it is
             // not a terminal level error like all other exception.
         } catch (final Exception e) {

uitest/src/main/java/com/vaadin/tests/components/upload/InterruptUploadWithPush.java

+package com.vaadin.tests.components.upload;
+
+import com.vaadin.annotations.Push;
+
+@Push
+public class InterruptUploadWithPush extends InterruptUpload {
+
+    @Override
+    protected Integer getTicketNumber() {
+        return 11616;
+    }
+
+    @Override
+    public String getDescription() {
+        return "Interrupting an upload with @Push shouldn't prevent uploading that same file immediately afterwards.";
+    }
+
+}

uitest/src/test/java/com/vaadin/tests/components/upload/InterruptUploadWithPushTest.java

+package com.vaadin.tests.components.upload;
+
+public class InterruptUploadWithPushTest extends InterruptUploadTest {
+}