fix: fix ServletFileUpload header encoding (#20480)

This change will set ServletFileUpload's header encoding always to UTF-8 in StreamReceiverHandler when request character encoding is null. This ensures that system's default character encoding is not applied when parsing filename of the uploaded file, unless request's character encoding is set otherwise.
Only for setups without multipart config for servlet.

Fixes: #20417

flow-server/src/main/java/com/vaadin/flow/server/communication/StreamReceiverHandler.java

@@ -630,11 +630,22 @@ protected Collection<Part> getParts(VaadinRequest request)
 
     protected FileItemInputIterator getItemIterator(VaadinRequest request)
             throws FileUploadException, IOException {
+        JakartaServletFileUpload upload = createServletFileUpload(request);
+        return upload.getItemIterator((HttpServletRequest) request);
+    }
+
+    // protected for testing purposes only
+    protected JakartaServletFileUpload createServletFileUpload(VaadinRequest request) {
         JakartaServletFileUpload upload = new JakartaServletFileUpload();
         upload.setSizeMax(requestSizeMax);
         upload.setFileSizeMax(fileSizeMax);
         upload.setFileCountMax(fileCountMax);
-        return upload.getItemIterator((HttpServletRequest) request);
+        if (request.getCharacterEncoding() == null) {
+            // Request body's file upload headers are expected to be encoded in
+            // UTF-8 if not explicitly set otherwise in the request.
+            upload.setHeaderCharset(StandardCharsets.UTF_8);
+        }
+        return upload;
     }
 
     public void setRequestSizeMax(long requestSizeMax) {

flow-server/src/test/java/com/vaadin/flow/server/communication/StreamReceiverHandlerTest.java

@@ -16,6 +16,7 @@
 import java.util.List;
 import java.util.Optional;
 
+import org.apache.commons.fileupload.servlet.ServletFileUpload;
 import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
@@ -87,6 +88,7 @@ public class StreamReceiverHandlerTest {
     private List<Part> parts;
 
     private boolean isGetContentLengthLongCalled;
+    private String requestCharacterEncoding;
 
     @Before
     public void setup() throws Exception {
@@ -181,6 +183,11 @@ public long getContentLengthLong() {
                 isGetContentLengthLongCalled = true;
                 return 0;
             }
+
+            @Override
+            public String getCharacterEncoding() {
+                return requestCharacterEncoding;
+            }
         };
     }
 
@@ -291,6 +298,27 @@ public void doHandleMultipartFileUpload_noPart_uploadFailed_responseStatusIs500_
         Assert.assertTrue(isGetContentLengthLongCalled);
     }
 
+    @Test
+    public void createServletFileUpload_useUTF8HeaderCharacterEncodingWhenRequestCharEncodingIsNotSet() {
+        ServletFileUpload servletFileUpload = handler
+                .createServletFileUpload(request);
+        Assert.assertNotNull(servletFileUpload);
+        Assert.assertEquals(
+                "Header encoding should be UTF-8 when request character encoding is null",
+                "UTF-8", servletFileUpload.getHeaderEncoding());
+    }
+
+    @Test
+    public void createServletFileUpload_dontSetHeaderCharEncodingWhenRequestCharEncodingIsSet() {
+        requestCharacterEncoding = "ASCII";
+        ServletFileUpload servletFileUpload = handler
+                .createServletFileUpload(request);
+        Assert.assertNotNull(servletFileUpload);
+        Assert.assertNull(
+                "Header encoding should not be set by Flow when request character encoding is set",
+                servletFileUpload.getHeaderEncoding());
+    }
+
     @Test
     public void doHandleMultipartFileUpload_hasParts_uploadFailed_responseStatusIs500()
             throws IOException {