title: File contents returned instead of file location link
body: <p>Models have this relation:</p>
  
  <pre><code>class ShoppingList(models.Model):
     (...)
     config_file = models.FileField(upload_to=upload_config_file)
  
  
  class FetchedData(models.Model):
      (...)
      config_id = models.ForeignKey(ShoppingList, on_delete=models.CASCADE, default=0)
  
      def config_link(self):
          return self.config_id.config_file
  </code></pre>
  
  <p>Then, in <code>serializers.py</code>:</p>
  
  <pre><code>class FetchedDataSerializer(serializers.ModelSerializer):
      file_link = serializers.SerializerMethodField()
  
      class Meta:
          model = FetchedData
          fields = ('config_id', 'file_link')
  
      def get_file_link(self, obj):
          return obj.config_link()
  </code></pre>
  
  <p>The problem is, when I get response, files are represented as their contents, so in case of JSON files I get something like:</p>
  
  <pre><code>{
      "config_id": "4544",
      "file_link": [
          "{\r\n",
          "    \"glossary\": {\r\n",
          "        \"title\": \"example glossary\",\r\n",
          "\t\t\"GlossDiv\": {\r\n",
          "            \"title\": \"S\",\r\n",
          "\t\t\t\"GlossList\": {\r\n",
          "                \"GlossEntry\": {\r\n",
          "                    \"ID\": \"SGML\",\r\n",
          "\t\t\t\t\t\"SortAs\": \"SGML\",\r\n",
          "\t\t\t\t\t\"GlossTerm\": \"Standard Generalized Markup Language\",\r\n",
          "\t\t\t\t\t\"Acronym\": \"SGML\",\r\n",
          "\t\t\t\t\t\"Abbrev\": \"ISO 8879:1986\",\r\n",
          "\t\t\t\t\t\"GlossDef\": {\r\n",
          "                        \"para\": \"A meta-markup language, used to create markup languages such as DocBook.\",\r\n",
          "\t\t\t\t\t\t\"GlossSeeAlso\": [\"GML\", \"XML\"]\r\n",
          "                    },\r\n",
          "\t\t\t\t\t\"GlossSee\": \"markup\"\r\n",
          "                }\r\n",
          "            }\r\n",
          "        }\r\n",
          "    }\r\n",
          "}"
      ]
  }
  </code></pre>
  
  <p>What to do to receive just a file location link instead?</p>