How do you set the file extension while uploading images through file_save_data?

I am trying to create a product with a product image on drupal using API. I have created a custom module for the same under which following code to upload my image to the Drupal:

$path = base64_decode($encodedData['content']);
$data= $this->grabImage($path);
$file = file_save_data($data, null, FileSystemInterface::EXISTS_REPLACE);
        
$id = $file->id();

I am using file_save_data drupal method to upload the image successfully and it gets uploaded to the following drupal directory:

mystore2/web/sites/default/files

Now, I am using this $file->id() //say 53 value in my product array to connect this image to my product.

$image['field_picture'] = [[
    "target_id" => 53,
    "alt" => 'abcd',
    "width"=> 224,
    "height"=> 225,
    "title" => '',
    "target_type" => "file"
]];

Follow response I am receiving from API:

> message:
>     Unprocessable Entity: validation failed.
>     field_picture.0: Only files with the following extensions are allowed: png gif jpg jpeg.\n

So, How can I pass extension in file_save_data()? Or how can I add this image to my product?

I am using Drupal version ^9.

You need to pass the file path, including its name and extension, as the $destination parameter:

$file = file_save_data($data, 'public://path/to/file.ext', FileSystemInterface::EXISTS_REPLACE);