text: Title: Exception caught for serializer field ReadOnlyField with a FileField source
  Body: **Describe the bug**
  
  I have a simple file server:
  
  ```python
  from django.db import models
  from django.shortcuts import render
  from rest_framework import generics, serializers
  
  class FooFile(models.Model):
      fname = models.FileField(max_length=1024, unique=True)
  
  class FooFileSerializer(serializers.HyperlinkedModelSerializer):
      fname = serializers.FileField(use_url=False, required=False)
      fsize = serializers.ReadOnlyField(source="fname.size")  # <-- herein lies the problem
  
      class Meta:
          model = FooFile
          fields = ("fname", "fsize")
  
  class FooFileList(generics.ListCreateAPIView):
      http_method_names = ["get", "post"]
      serializer_class = FooFileSerializer
      queryset = FooFile.objects.all()
  ```
  
  Generating the OpenAPI schema using drf-spectacular prints this warning:
  
  ```
   Warning [FooFileList > FooFileSerializer]: could not resolve field on model <class 'foo.models.FooFile'> with path "fname.size". This is likely a custom field that does some unknown magic. Maybe consider annotating the field/property? Defaulting to "string". (Exception: 'NoneType' object has no attribute '_meta')
  ```
  
  **To Reproduce**
  
  See https://github.com/jennydaman/spectacular-fsize-bug
  
  **Expected behavior**
  
  No warnings. `components.schemas.FooFile.properties.fsize.type` should be `number`.
  