requests_toolbelt.MultipartEncoder fails to encode S3File objects due to missing `len` property

Hello,

I'm unable to use a S3File object together with requests_toolbelt.MultipartEncoder (https://toolbelt.readthedocs.io/en/latest/user.html). The MultipartEncoder expects the File object to contain len property to calculate its size and fails with the following:

File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 125, in __init__
    self._prepare_parts()
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 246, in _prepare_parts
    self.parts = [Part.from_field(f, enc) for f in self._iter_fields()]
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 246, in <listcomp>
    self.parts = [Part.from_field(f, enc) for f in self._iter_fields()]
                  ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 495, in from_field
    return cls(headers, body)
           ^^^^^^^^^^^^^^^^^^
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 488, in __init__
    self.len = len(self.headers) + total_len(self.body)
                                   ^^^^^^^^^^^^^^^^^^^^
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 432, in total_len
    if hasattr(o, 'len'):
       ^^^^^^^^^^^^^^^^^
  File "/home/krzysztof/venvs/py311/lib/python3.11/site-packages/requests_toolbelt/multipart/encoder.py", line 573, in len
    return total_len(self.fd) - self.fd.tell()
           ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

It works with a custom adapter like this:

class S3FileWithLenAdapter:
    def __init__(self, s3_file):
        self._s3_file = s3_file

    @property
    def len(self):
        return self._s3_file.size

    def __getattr__(self, name):
        return getattr(self._s3_file, name)

Could you consider adding len property to S3File objects?

Start by locating the S3File implementation and reviewing its existing size handling, then compare it with requests_toolbelt/multipart/encoder.py, especially total_len and the S3File-related len access shown in the traceback. Done means MultipartEncoder can consume an S3File without the reported TypeError; add or update focused tests if the repository provides them.