Impossible to upload SVG via the webapi.

It fails due to vendor/magento/framework/Api/ImageContentValidator.php:55

```
$imageProperties = @getimagesizefromstring($fileContent);
if (empty($imageProperties)) {
    throw new InputException(new Phrase('The image content must be valid base64 encoded data.'));
}
```

SVG content is not compatible with the getimagesizefromstring php function. It needs to get a specific validation.