Does service module expect Base64 encoding for file uploads through http-post (drupal 7)?
Short answer yes it does. There is a create raw method but unfortunately, it doesn't work very well b/c drupal 7s file handling is a joke.

Ive created a patch that will allow you to attach files to  a node and it uses multipart form, but its a work in progress.

http://drupal.org/node/1484992