fix: Deduce content type from file name (#21611)

Author: mshabarov

flow-server/src/main/java/com/vaadin/flow/server/streams/DownloadResponse.java

@@ -52,16 +52,17 @@ public class DownloadResponse implements Serializable {
      *            Content-Disposition header to 'attachment' if the value is not
      *            <code>null</code>, otherwise the header is not set
      * @param contentType
-     *            content type
+     *            content type or a value determined from fileName if
+     *            {@code null}
      * @param contentLength
      *            byte size of a stream or <code>-1</code> if unknown
      */
     public DownloadResponse(InputStream inputStream, String fileName,
             String contentType, long contentLength) {
         this.inputStream = inputStream;
         this.fileName = fileName;
-        this.contentType = contentType;
         this.contentLength = contentLength;
+        this.contentType = contentType;
     }
 
     /**
@@ -86,6 +87,10 @@ public String getFileName() {
 
     /**
      * Get the content type.
+     * <p>
+     * For a {@code null} value the type should be gotten from
+     * {@code VaadinService.getMimeType(fileName)} or be set to default value
+     * {@code application/octet-stream}
      *
      * @return content type
      */

flow-server/src/main/java/com/vaadin/flow/server/streams/InputStreamDownloadHandler.java

@@ -55,7 +55,11 @@ public void handleDownloadRequest(DownloadEvent downloadEvent)
         }
 
         String downloadName = download.getFileName();
-        downloadEvent.setContentType(getContentType(downloadName, response));
+        String contentType = download.getContentType() == null
+                ? getContentType(downloadName, response)
+                : download.getContentType();
+        downloadEvent.setContentType(contentType);
+
         if (!isInline()) {
             downloadEvent.setFileName(downloadName);
         }

flow-server/src/test/java/com/vaadin/flow/server/streams/InputStreamDownloadHandlerTest.java

@@ -252,6 +252,50 @@ public void inputStreamDownloadCallback_doesNotRequireCatch() {
         });
     }
 
+    @Test
+    public void downloadResponseHasContentType_contentTypeUsed()
+            throws IOException {
+        String contentType = "custom";
+        InputStreamDownloadHandler handler = new InputStreamDownloadHandler(
+                event -> {
+                    InputStream stream = Mockito.mock(InputStream.class);
+                    Mockito.when(stream.read(Mockito.any(), Mockito.anyInt(),
+                            Mockito.anyInt())).thenReturn(-1);
+                    return new DownloadResponse(stream, "report.pdf",
+                            contentType, 0);
+                });
+
+        DownloadEvent event = new DownloadEvent(request, response, session,
+                new Element("div"));
+
+        handler.handleDownloadRequest(event);
+
+        Mockito.verify(response).setContentType(contentType);
+    }
+
+    @Test
+    public void downloadResponseNullContentType_fileTypeIsUsed()
+            throws IOException {
+        String contentType = "file/pdf";
+
+        Mockito.when(service.getMimeType("report.pdf")).thenReturn(contentType);
+
+        InputStreamDownloadHandler handler = new InputStreamDownloadHandler(
+                event -> {
+                    InputStream stream = Mockito.mock(InputStream.class);
+                    Mockito.when(stream.read(Mockito.any(), Mockito.anyInt(),
+                            Mockito.anyInt())).thenReturn(-1);
+                    return new DownloadResponse(stream, "report.pdf", null, 0);
+                });
+
+        DownloadEvent event = new DownloadEvent(request, response, session,
+                new Element("div"));
+
+        handler.handleDownloadRequest(event);
+
+        Mockito.verify(response).setContentType(contentType);
+    }
+
     private static byte[] getBytes() {
         // Simulate a download of 165000 bytes
         byte[] data = new byte[165000];