[FIX] demo: missing waiting on image upload

The route `upload-image` in the demo server wasn't waiting for the
stream to write the whole image to a file before sending the response.

This led to corrupted images when trying to create a figure with a
large image.

closes #6116

Task: 0
Signed-off-by: Rémi Rahir (rar) <rar@odoo.com>

Author: hokolomopo

tools/server/main.cjs

@@ -112,8 +112,10 @@ app.post("/upload-image", function (req, res) {
         // This opens up the writeable stream to `output`
         const writeStream = fs.createWriteStream(output);
         // This pipes the POST data to the file
+        writeStream.on("finish", () => {
+          res.send("../" + output);
+        });
         readStream.pipe(writeStream);
-        res.send("../" + output);
       }
     });
   } catch (err) {