Posting Django FileFields before they're saved

I'm having issues when trying to POST Django FileFields files before they're saved. I'm trying to post them during a model save override (to get extra data). So in MyModel.save override it looks like this:

class MyModel(models.Model):
    image = models.ImageField()
    def save(self, *args, **kwargs):
        res = unirest.post(url, headers=headers, params={
            "image_request[image]": self.image.file
        })
        # @todo: add some field from res.body to my model.
        super(MyModel, self).save(*args, **kwargs)

Problem is, before the model is saved, self.image.file is either an instance of InMemoryUploadedFile or TemporaryUploadedFile. When I have the latter (or after model is saved), I have to pass in open(self.image.file.path), as self.image.file.open() won't work. I wonder why.

Start by comparing Django's InMemoryUploadedFile and TemporaryUploadedFile behavior in django/core/files/uploadedfile.py with unirest's file-upload request handling. Determine whether the client documents or supports these file-like objects, and define completion as a clear explanation or a reproducible fix for posting unsaved FileFields.