wasm: actually save dropped files to /qt/tmp

We were reporting the correct url, but saving to the
root of the file system.

Pick-to: 6.7
Change-Id: I3e1fd63efb77dd75ccf7a8f44a3211965fe11be8
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/plugins/platforms/wasm/qwasmdom.cpp

@@ -170,7 +170,7 @@ void DataTransfer::toMimeDataWithFile(std::function<void(QMimeData *)> callback)
                 QUrl fileUrl = QUrl::fromLocalFile(qtTmpDir.filePath(QString::fromStdString(webfile.name())));
                 mimeContext->fileUrls.append(fileUrl);
 
-                QFile file(fileName);
+                QFile file(fileUrl.toLocalFile());
                 if (!file.open(QFile::WriteOnly)) {
                     qWarning() << "File was not opened";
                     mimeContext->deref();