MIME type not set when file uploaded with REST API

Sorry if this is an issue for the REST module rather than file_entity, but I could not upload files at all using the REST API until I downloaded and enabled this module.

The issue I'm experiencing is that while I can create new file entities by POSTing a base64 encoded file, the filemime setting is ignored and is always set to "application/octet-stream":

curl --include --request POST 
--header 'Content-type: application/hal+json' 
http://site.dev/entity/file?_format=hal+json 
--data-binary
{
  "_links": {
    "type": {
      "href": "http://site.dev/rest/type/file/image"
    }
  },
  "filename": [{"value": "file.png"}],
  "filemime": [{"value": "image/png"}],
  "type": [{"target_id": "image"}],
  "data": [{
    "value": "iVBORw0KGgoAAAANSUhEUgAAACAAAAAgBAMAAACBVGfHAAAABGdBTUEAALGPC/xhBQAAAAFzUkdCAK7OHOkAAAAJcEhZcwAACxMAAAsTAQCanBgAAAAkUExURUxpcSOq4SOq4SOq4SOq4SOq4SOq4SOq4SOq4SOq4SOq4SOq4U0m8UcAAAAMdFJOUwD/EvAm2qhTPMRrkl4CMYAAAAE1SURBVCjPXZG/S8NQEMePR7Ehnb48khC7BEFFnYJD6/aWOLilVBy6WBVc648/oCii1KlFKDrFwV2tqH+edy9Rk7zh8e7Dfe++944oP5dUO59RDQyPqrFK/WqKCzxUwA30oJyymervlVK8PsVBOd85hh6pyflfhwXg024cjguwBqCjhsCsSOAn+k4MBGpUWAB6Tb6W1VxAgv+MfcPgWUBgazjxoYwlQLrsmBa6MhbH3hep2yu6QJuowWL9Yfs1U4QRSbWOjRts0DMCdEbqjl8s1kYkPqmX7YGRfgy4aJe2IB+0wZJI2s7ojEGbWtJejL1aL55h+wGDVWTWix47+cAug6kAycjs+LkkjFzku0h6dC3D01KxCmdOLrt7pyQ0+Z/tRXT/eEr0dvL7y6zkrbhPtYVPrOAHKZQ53IyepQMAAAAASUVORK5CYII="
  }]
}

Response:

{
   "fid":[
      {
         "value":"10"
      }
   ],
   "uuid":[
      {
         "value":"d57fc72e-c527-4c70-a4ad-820050506a5d"
      }
   ],
   "langcode":[
      {
         "value":"en"
      }
   ],
   "uid":[

   ],
   "filename":[
      {
         "value":"image.png"
      }
   ],
   "uri":[
      {
         "value":"public:\/\/file6GF0Fz"
      }
   ],
   "filemime":[
      {
         "value":"application\/octet-stream"
      }
   ],
   "filesize":[
      {
         "value":488
      }
   ],
   "status":[
      {
         "value":false
      }
   ],
   "created":[
      {
         "value":1466642149
      }
   ],
   "changed":[
      {
         "value":1466642149
      }
   ],
   "type":[
      {
         "target_id":"image",
         "target_type":"file_type",
         "target_uuid":"c4ab5ab7-b9ad-4f7e-946d-17d8c397b50d"
      }
   ],
   "field_image_alt_text":[

   ],
   "field_image_title_text":[

   ]
}