There's no user_id indication in the file structure.
It's all based on the attachment ID.
Attachments with IDs 0-999 are stored in the 0 directory. 1000-1999 stored in the 1 directory etc.
The files in internal_data/attachments are the full bits of the attachment. Whereas the files in the data/attachments are the thumbnails (also stored in the same numbered directory format).