How to mock f from with open as f being passed to a function?

Question

I have a function that opens a file within a context manager and then passes its handler to a method of an object.

with open("somefile.txt", "rb") as f:
    some_obj.upload(file=f)

My question is how can I unit test pass that f handler? I would like to create a mock open and a mock of some_obj.upload, and check that the file parameter was actually given f.

Currently, I have something like this:

def upload_local_file(local_file: str):
    my_obj = SOMEOBJ()
    with open(local_file, "rb") as f:
        my_obj.upload(file=f)

@mock.patch("some_obj")
@mock.patch("builtins.open")
def test_upload_local_file_positive(mock_open, mock_obj):
    f = mock_open.return_value
    f.return_value = "/tmp/example_file.txt"

    upload_local_file(local_file="/tmp/example_file.txt")

    mock_open.assert_called_once_with("/tmp/example_file.txt", "rb")
    mock_obj.return_value.upload.assert_called_once_with(
        path="/tmp/example_file.txt",
        file=f
    )

For some reason, it's failing because the file differs in some ways I don't understand, please help me fix this unit test

>       mock_obj.return_value.upload.assert_called_once_with(
            path="/tmp/example_file.txt",
            file=f
        )
E       AssertionError: expected call not found.
E       Expected: upload(path='/tmp/example_file.txt', file=<MagicMock name='open()' id='4432144032'>)
E       Actual: upload(file=<MagicMock name='open().__enter__()' id='4432212560'>)
E       
E       pytest introspection follows:
E       
E       Kwargs:
E       assert {'file': <MagicMock name='open().__enter__()' id='4432212560'>} == {'file': <MagicMock name='open()' id='4432144032'>,\n 'path': '/tmp/example_file.txt'}
E         Differing items:
E         {'file': <MagicMock name='open().__enter__()' id='4432212560'>} != {'file': <MagicMock name='open()' id='4432144032'>}
E         Right contains 1 more item:
E         {'path': '/tmp/example_file.txt'}
E         Full diff:
E           {
E         -  'file': <MagicMock name='open()' id='4432144032'>,
E         ?                                            ^^ --
E         +  'file': <MagicMock name='open().__enter__()' id='4432212560'>,
E         ?                                 ++++++++++++          + ^^^
E         -  'path': '/tmp/example_file.txt',
E           }