How do I migrate file uploads?
Your custom_upload migration is trying to create nodes; the custom_upload migration is taking a d6_upload source plugin:

source:
  plugin: d6_upload


...and mapping it to a node:entity destination plugin:

destination:
  plugin: 'entity:node'


Since the d6_upload source doesn't have a title field to map to the node:entity field in the process definition of your migration, the migration fails as it tries to insert a titleless node.

You need update your node migration. If your node/file migrations are working, the node migration should have a 
process.field_attachments yml property to map field_attachments.target_id to the fid property from your custom_file migration.