Calling `.reload()` on an object before `FileField.replace` breaks the FileField

Setup:

from mongoengine.document import Document
from mongoengine.fields import StringField, FileField
import pickle
import pandas as pd

class TestObject(Document):
    name = StringField(primary_key=True)
    file = FileField()

testdata1 = pd.DataFrame([[1, 2, 3], [0, 4, 6], [9, 3, 5]], columns=['A', 'B', 'C'])
testdata2 = pd.DataFrame([[5, 6, 7], [2, 4, 8], [0, 6, 1]], columns=['A', 'B', 'C'])

Minimal working example:

db_obj = TestObject(name='test1')
db_obj.file = pickle.dumps(testdata1)
db_obj.save()
db_obj.file.replace(pickle.dumps(testdata2))
db_obj.save()
print(pickle.loads(TestObject.objects().get(name='test1').file.read()))

Prints:

   A  B  C
0  5  6  7
1  2  4  8
2  0  6  1

Minimal broken example:

db_obj = TestObject(name='test2')
db_obj.file = pickle.dumps(testdata1)
db_obj.save()
db_obj.reload()  ##
db_obj.save()  ##
db_obj.file.replace(pickle.dumps(testdata2))
db_obj.save()
print(pickle.loads(TestObject.objects().get(name='test2').file.read()))

Raises:

TypeError: a bytes-like object is required, not 'NoneType'

I compared the file ids for the local and DB objects at each step using:

print(db_obj.file, TestObject.objects().get(name='test1').file)

In both cases, the file IDs stay the same until the .replace:

<GridFSProxy: None (5f8cdc7ed4010827a9cac125)> <GridFSProxy: None (5f8cdc7ed4010827a9cac125)>

After the .replace, the database loses its file, while the local object has a new one:

<GridFSProxy: None (5f8cdc7ed4010827a9cac127)> <GridFSProxy: <no file> (5f8cdc7ed4010827a9cac125)>

In the working example, the .save() resolves this discrepancy:

<GridFSProxy: None (5f8cdc7ed4010827a9cac127)> <GridFSProxy: None (5f8cdc7ed4010827a9cac127)>

But in the broken example, there is no change after .save(), and the file ID discrepancy cannot be resolved. If other fields are included on the object and modified, they update properly with that final .save(), but the FileField does not.

Start by reproducing the minimal broken example with FileField, reload(), save(), and replace(), then inspect the FileField and GridFSProxy behavior across those calls. Add a regression test covering the reload-before-replace sequence and verify that the replacement file remains available after the final save.