QueryDict.copy() returns closed files when the type of file is TemporaryUploadedFile

[Liquid Scorpio]

When uploaded file size is greater than FILE_UPLOAD_MAX_MEMORY_SIZE, Django uses TemporaryUploadedFile to represent the file object. However, when executing .copy() on a QueryDict containing such a file, the returned object has the file but it is in closed state (seekable() is False).

Expected: File should be present in open state (seekable() should be True)

Below is a reproducible example and also contains version details:

Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
In [18]: import django

In [19]: django.VERSION
Out[19]: (1, 11, 11, u'final', 0)

In [20]: from django.http.request import QueryDict

In [21]: from django.core.files.uploadedfile import TemporaryUploadedFile

In [22]: d = QueryDict(mutable=True)

In [23]: f = TemporaryUploadedFile('test.jpg', 'image/jpeg', 100, 'utf-8')

In [24]: f.seekable()
Out[24]: True

In [25]: d.appendlist('image', f)

In [26]: d['image'].seekable()
Out[25]: True

In [27]: c = d.copy()

In [28]: c['image'].seekable()
Out[28]: False

In [30]: 

[Tim Graham]

I can reproduce with Python 2, even if I'm unsure that the behavior is incorrect:

>>> from django.core.files.uploadedfile import TemporaryUploadedFile
>>> from copy import deepcopy
>>> f = TemporaryUploadedFile('test.jpg', 'image/jpeg', 100, 'utf-8')
f.seekable()
True
a = deepcopy(f)
>>> a.seekable()
False

However, the deepcopy() crashes on Python 3 with:
TypeError: cannot serialize '_io.BufferedRandom' object

Accepting for further investigation.