Check for presence of os.unlink in temp.py

I don't think this is really the solution, since Guido's point is really the root cause.

It sounds like we need a setting for the file-size management that says "never, ever save to disk" and then if you've set that and somebody uploads something that eats all your memory, that's your problem. Either we can write to disk (in which case os.unlink has to exist so we can manage the files properly) or we can't, in which case we should allow the user to specify that. Transparently moving into that "don't write to disk mode" would be dangerous, however, since it really does run the risk of a small denial of service style attack so the person setting up the applications must agree to that explicitly.

One option might be to defer the importing of any write-specific module initialisations, or at least import these conditionally.

For example, the user should only specify to use the MemoryFileUploadHandler and not the TemporaryFileUploadHandler in their settings (something that's not currently done in the rietveld project):

FILE_UPLOAD_HANDLERS = (
    'django.core.files.uploadhandler.MemoryFileUploadHandler',
    )

And then in core/files/uploadedfile.py, delay the importing until the TemporaryUploadedFile constructor:

class TemporaryUploadedFile(UploadedFile):
    """
    A file uploaded to a temporary location (i.e. stream-to-disk).
    """
    def __init__(self, name, content_type, size, charset):
        super(TemporaryUploadedFile, self).__init__(name, content_type, size, charset)
        # Moved this from top of file
        from django.core.files import temp as tempfile
        if settings.FILE_UPLOAD_TEMP_DIR:
            self._file = tempfile.NamedTemporaryFile(suffix='.upload', dir=settings.FILE_UPLOAD_TEMP_DIR)
        else:
            self._file = tempfile.NamedTemporaryFile(suffix='.upload')

Or, only import temp if TemporaryFileUploadHandler is in settings.FILE_UPLOAD_HANDLERS:

from django.conf import settings
from django.core.files.base import File
from django.utils.encoding import smart_str

if 'django.core.files.uploadhandler.TemporaryFileUploadHandler' in settings.FILE_UPLOAD_HANDLERS:
    from django.core.files import temp as tempfile

Both of these methods work for me (I can attach patches for either/both if you like, however I suspect they're still a little hacky).