Constantly getting "The website encountered an unexpected error. Please try again later."
That exception is thrown by EntityTypeManager::getDefinition() when it doesn't find any definition for the entity with the ID passed as argument.

  if (($entity_type = parent::getDefinition($entity_type_id, FALSE)) &amp;&amp; class_exists($entity_type-&gt;getClass())) {
    return $entity_type;
  }
  elseif (!$exception_on_invalid) {
    return NULL;
  }

  throw new PluginNotFoundException($entity_type_id, sprintf('The "%s" entity type does not exist.', $entity_type_id));


The ID of that entity could confuse: It's not the File entity Drupal core implements because its ID would be file, not file_type. That is the entity defined from the File Entity (fieldable files) project, in its FileType.php file.

I take you installed the module, and uninstalled it, but your site still have some instances of that configuration entity. I would try cleaning Drupal cache, and if that doesn't work, I would re-install the module, go to /admin/structure/file-types, delete any entity listed there, and uninstall the module again.