If you add a custom class to the file uploader, you should be able to hide the button. Add the custom class, find the download button class with f12 inspector and add the class of the button to your custom class. Will look like this in your sass file;
.YourCustomClass {
.buttonClass {
display: none;
}
}
You can follow this learning path to learn how to apply custom styling to your app:
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass