How to disable a file upload button for anonymous users?
The Field permissions module should help you achieve this.


  The Field Permissions module allows site administrators to set
   field-level permissions to edit, view and create fields on any entity.
  
  Enable field permissions on any entity, not just nodes.
  
  Role-based field permissions allowing different viewing patterned
  based on what access the user has.
  
  Author-level permissions allow viewing and editing of fields based on
  who the entity owner is.
  
  Permissions for each field are not enabled by default. Instead,
  administrators can enable these permissions explicitly for the fields
  where this feature is needed.