Theme file upload form element
As suggested by Ivan Jaros, you can override input--file.html.twig if you want to use twig templates. Basic input template is as follow:

{#
/**
 * @file
 * Theme override for an 'input' #type form element.
 *
 * Available variables:
 * - attributes: A list of HTML attributes for the input element.
 * - children: Optional additional rendered elements.
 *
 * @see template_preprocess_input()
 */
#}
&lt;input{{ attributes }} /&gt;{{ children }}


If you need to add more dynamic variables, you can use template_preprocess_input() preprocess function.

Also, you can use debug: true under twig.config in services.yml. This will help for theme-related development.

Note: Drupal provides default classes that are from generic to specific to the field. If generic classes helps to theme, I would suggest to avoid all the above.