Validator for files, checking the size, extension and mimetype. Modified to work with Django 2.0.6 and Python 3.6.4

validators.py

from os.path import splitext

import mimetypes
from django.core.exceptions import ValidationError
from django.template.defaultfilters import filesizeformat
from django.utils.translation import ugettext_lazy as _
from django.utils.deconstruct import deconstructible


@deconstructible
class FileValidator(object):
    """
    Validator for files, checking the size, extension and mimetype.
    
    Modified to work with Django 2.0.6 and Python 3.6.4

    Initialization parameters:
        allowed_extensions: iterable with allowed file extensions
            ie. ('txt', 'doc')
        allowd_mimetypes: iterable with allowed mimetypes
            ie. ('image/png', )
        min_size: minimum number of bytes allowed
            ie. 100
        max_size: maximum number of bytes allowed
            ie. 24*1024*1024 for 24 MB

    Usage example::

        MyModel(models.Model):
            myfile = FileField(validators=FileValidator(max_size=24*1024*1024), ...)

    """

    extension_message = _("Extension '%(extension)s' not allowed. Allowed extensions are: '%(allowed_extensions)s.'")
    mime_message = _("MIME type '%(mimetype)s' is not valid. Allowed types are: %(allowed_mimetypes)s.")
    min_size_message = _('The current file %(size)s, which is too small. The minumum file size is %(allowed_size)s.')
    max_size_message = _('The current file %(size)s, which is too large. The maximum file size is %(allowed_size)s.')

    def __init__(self, *args, **kwargs):
        self.allowed_extensions = kwargs.pop('allowed_extensions', None)
        self.allowed_mimetypes = kwargs.pop('allowed_mimetypes', None)
        self.min_size = kwargs.pop('min_size', 0)
        self.max_size = kwargs.pop('max_size', None)

    def __call__(self, value):
        """
        Check the extension, content type and file size.
        """

        # Check the extension
        ext = splitext(value.name)[1][1:].lower()
        if self.allowed_extensions and ext not in self.allowed_extensions:
            message = self.extension_message % {
                'extension': ext,
                'allowed_extensions': ', '.join(self.allowed_extensions)
            }

            raise ValidationError(message)

        # Check the content type
        mimetype = mimetypes.guess_type(value.name)[0]
        if self.allowed_mimetypes and mimetype not in self.allowed_mimetypes:
            message = self.mime_message % {
                'mimetype': mimetype,
                'allowed_mimetypes': ', '.join(self.allowed_mimetypes)
            }

            raise ValidationError(message)

        # Check the file size
        filesize = len(value)
        if self.max_size and filesize > self.max_size:
            message = self.max_size_message % {
                'size': filesizeformat(filesize),
                'allowed_size': filesizeformat(self.max_size)
            }

            raise ValidationError(message)

        elif filesize < self.min_size:
            message = self.min_size_message % {
                'size': filesizeformat(filesize),
                'allowed_size': filesizeformat(self.min_size)
            }

            raise ValidationError(message)