Because input file validation is a common step used widely throughout app APIs, it is more efficient to create a separate validator and reuse it.

Writing a validator

Requirements

pip install python-magic : pip install python-magic-bin==0.4.14 automatically fulfills libmagic dependency. However, it is highly likely to cause dependency issue with various AWS features. Hence, go with python-magic

brew install libmagic : This is required if you use python-magic, and an alternative (supposedly) pip install python-libmagic hasn't worked for me for some unidentified reason.

Code Snippet from Stackoverflow

In validators.py,

import magic

from django.utils.deconstruct import deconstructible
from django.template.defaultfilters import filesizeformat


@deconstructible
class FileValidator(object):
    error_messages = {
     'max_size': ("Ensure this file size is not greater than %(max_size)s."
                  " Your file size is %(size)s."),
     'min_size': ("Ensure this file size is not less than %(min_size)s. "
                  "Your file size is %(size)s."),
     'content_type': "Files of type %(content_type)s are not supported.",
    }

    def __init__(self, max_size=None, min_size=None, content_types=()):
        self.max_size = max_size
        self.min_size = min_size
        self.content_types = content_types

    def __call__(self, data):
        if self.max_size is not None and data.size > self.max_size:
            params = {
                'max_size': filesizeformat(self.max_size), 
                'size': filesizeformat(data.size),
            }
            raise ValidationError(self.error_messages['max_size'],
                                   'max_size', params)

        if self.min_size is not None and data.size < self.min_size:
            params = {
                'min_size': filesizeformat(self.min_size),
                'size': filesizeformat(data.size)
            }
            raise ValidationError(self.error_messages['min_size'], 
                                   'min_size', params)

        if self.content_types:
            content_type = magic.from_buffer(data.read(), mime=True)
            data.seek(0)

            if content_type not in self.content_types:
                params = { 'content_type': content_type }
                raise ValidationError(self.error_messages['content_type'],
                                   'content_type', params)

    def __eq__(self, other):
        return (
            isinstance(other, FileValidator) and
            self.max_size == other.max_size and
            self.min_size == other.min_size and
            self.content_types == other.content_types
        )

Usage in Serializers.py

validate_file = FileValidator(max_size=1024 * 100, 
                             content_types=('application/xml',))
file = models.FileField(upload_to=settings.XML_ROOT, 
                        validators=[validate_file])