node js sharp weird image after resizing

after i resize image, image’s quality is low and weird mono color

sharp(req.files.file[0].path).resize({height:250})
                                  .withMetadata()
                                  .toFile(`resize/${file_server}`, (err,info) =>{
                                    if(err) throw err;
                                    console.log(`info : ${info}`);
                                  })
                                  .toBuffer();
                                } else {req.files.file[0].filename = file_server}
                                console.log(req.files.file[0].mimetype.indexOf('image'))

before resizing file

enter image description here

this is a image after resizing

enter image description here

i dont know why my image return like this.

also .png file’s resizing is just black square return