This is an example of how to post a file attachment to a SharePoint 2013 list item. You must have the list contextinfo token available prior to adding attachments to a list item.

addAttachment.coffee

user     = "domain\\username"
pass     = "password"
url      = "http://sharepoint/siteCollection"
list     = "listName"
spItemId = 1
fileName = "filename.file"
data     = "" # The file data should be pulled from the fs using { encoding: null }
 
# This is pulled from REST using ./_api/contextinfo
context  = "some really long hex string value"

# Define the request fn
request = require 'request'

addAttachment: (cb)->
  proccessRequest = (err, res, body)->
    if err
      console.log err
  
    jsonBody = JSON.parse(body)
  
    if jsonBody.error && jsonBody.error.code.indexOf("Microsoft.SharePoint.Client.InvalidClientQueryException") >= 0
      cb(jsonBody.error, null)
  
    if jsonBody.error && jsonBody.error.code.indexOf("Microsoft.SharePoint.SPException")
      cb(jsonBody.error, null)
  
    else
      cb(err, JSON.parse(body).d)
  
  config =
    headers :
      "Accept": "application/json;odata=verbose"
      "X-RequestDigest": context
      "content-type": "application/json;odata=verbose"
    url     : "#{@url}/_api/web/lists/getbytitle('#{listName}')/items(#{spItemId})/AttachmentFiles/add(Filename='#{fileName}')"
    body: data
    binaryStringRequestBody: true
    state: "Update"
  
  request.post(config, processRequest).auth(user, pass, true)
  
addAttachment (err, res)->
  console.log err || res