[Django]-Only accept a certain file type in FileField, server-side

Django in version 1.11 has a newly added FileExtensionValidator for model fields, the docs is here: https://docs.djangoproject.com/en/dev/ref/validators/#fileextensionvalidator.

An example of how to validate a file extension:

from django.core.validators import FileExtensionValidator
from django.db import models


class MyModel(models.Model):
    pdf_file = models.FileField(
        upload_to="foo/", validators=[FileExtensionValidator(allowed_extensions=["pdf"])]
    )

Note that this method is not safe. Citation from Django docs:

Don’t rely on validation of the file extension to determine a file’s
type. Files can be renamed to have any extension no matter what data
they contain.

There is also new validate_image_file_extension for validating image extensions (using Pillow). Another option is to use ImageField in such a case, which also enforces the check that a valid image has been uploaded.

A few people have suggested using python-magic to validate that the file actually is of the type you are expecting to receive. This can be incorporated into the validator suggested in the accepted answer:

import os
import magic
from django.core.exceptions import ValidationError

def validate_is_pdf(file):
    valid_mime_types = ['application/pdf']
    file_mime_type = magic.from_buffer(file.read(1024), mime=True)
    if file_mime_type not in valid_mime_types:
        raise ValidationError('Unsupported file type.')
    valid_file_extensions = ['.pdf']
    ext = os.path.splitext(file.name)[1]
    if ext.lower() not in valid_file_extensions:
        raise ValidationError('Unacceptable file extension.')

This example only validates a pdf, but any number of mime-types and file extensions can be added to the arrays.

Assuming you saved the above in validators.py you can incorporate this into your model like so:

from myapp.validators import validate_is_pdf

class PdfFile(models.Model):
    file = models.FileField(upload_to='pdfs/', validators=(validate_is_pdf,))

You can use the below to restrict filetypes in your Form

file = forms.FileField(widget=forms.FileInput(attrs={'accept':'application/pdf'}))

There’s a Django snippet that does this:

import os

from django import forms

class ExtFileField(forms.FileField):
    """
    Same as forms.FileField, but you can specify a file extension whitelist.

    >>> from django.core.files.uploadedfile import SimpleUploadedFile
    >>>
    >>> t = ExtFileField(ext_whitelist=(".pdf", ".txt"))
    >>>
    >>> t.clean(SimpleUploadedFile('filename.pdf', 'Some File Content'))
    >>> t.clean(SimpleUploadedFile('filename.txt', 'Some File Content'))
    >>>
    >>> t.clean(SimpleUploadedFile('filename.exe', 'Some File Content'))
    Traceback (most recent call last):
    ...
    ValidationError: [u'Not allowed filetype!']
    """
    def __init__(self, *args, **kwargs):
        ext_whitelist = kwargs.pop("ext_whitelist")
        self.ext_whitelist = [i.lower() for i in ext_whitelist]

        super(ExtFileField, self).__init__(*args, **kwargs)

    def clean(self, *args, **kwargs):
        data = super(ExtFileField, self).clean(*args, **kwargs)
        filename = data.name
        ext = os.path.splitext(filename)[1]
        ext = ext.lower()
        if ext not in self.ext_whitelist:
            raise forms.ValidationError("Not allowed filetype!")

#-------------------------------------------------------------------------

if __name__ == "__main__":
    import doctest, datetime
    doctest.testmod()

First. Create a file named formatChecker.py inside the app where the you have the model that has the FileField that you want to accept a certain file type.

This is your formatChecker.py:

from django.db.models import FileField
from django.forms import forms
from django.template.defaultfilters import filesizeformat
from django.utils.translation import ugettext_lazy as _

class ContentTypeRestrictedFileField(FileField):
    """
    Same as FileField, but you can specify:
        * content_types - list containing allowed content_types. Example: ['application/pdf', 'image/jpeg']
        * max_upload_size - a number indicating the maximum file size allowed for upload.
            2.5MB - 2621440
            5MB - 5242880
            10MB - 10485760
            20MB - 20971520
            50MB - 5242880
            100MB 104857600
            250MB - 214958080
            500MB - 429916160
"""
def __init__(self, *args, **kwargs):
    self.content_types = kwargs.pop("content_types")
    self.max_upload_size = kwargs.pop("max_upload_size")

    super(ContentTypeRestrictedFileField, self).__init__(*args, **kwargs)

def clean(self, *args, **kwargs):        
    data = super(ContentTypeRestrictedFileField, self).clean(*args, **kwargs)

    file = data.file
    try:
        content_type = file.content_type
        if content_type in self.content_types:
            if file._size > self.max_upload_size:
                raise forms.ValidationError(_('Please keep filesize under %s. Current filesize %s') % (filesizeformat(self.max_upload_size), filesizeformat(file._size)))
        else:
            raise forms.ValidationError(_('Filetype not supported.'))
    except AttributeError:
        pass        

    return data

Second. In your models.py, add this:

from formatChecker import ContentTypeRestrictedFileField

Then instead of using ‘FileField’, use this ‘ContentTypeRestrictedFileField’.

Example:

class Stuff(models.Model):
    title = models.CharField(max_length=245)
    handout = ContentTypeRestrictedFileField(upload_to='uploads/', content_types=['video/x-msvideo', 'application/pdf', 'video/mp4', 'audio/mpeg', ],max_upload_size=5242880,blank=True, null=True)

Those are the things you have to when you want to only accept a certain file type in FileField.

after I checked the accepted answer, I decided to share a tip based on Django documentation. There is already a validator for use to validate file extension. You don’t need to rewrite your own custom function to validate whether your file extension is allowed or not.

https://docs.djangoproject.com/en/3.0/ref/validators/#fileextensionvalidator

Warning

Don’t rely on validation of the file extension to determine a file’s
type. Files can be renamed to have any extension no matter what data
they contain.

I think you would be best suited using the ExtFileField that Dominic Rodger specified in his answer and python-magic that Daniel Quinn mentioned is the best way to go. If someone is smart enough to change the extension at least you will catch them with the headers.

You can define a list of accepted mime types in settings and then define a validator which uses python-magic to detect the mime-type and raises ValidationError if the mime-type is not accepted. Set that validator on the file form field.

The only problem is that sometimes the mime type is application/octet-stream, which could correspond to different file formats. Did someone of you overcome this issue?

Additionally i Will extend this class with some extra behaviour.

class ContentTypeRestrictedFileField(forms.FileField):
    ...
    widget = None
    ...
    def __init__(self, *args, **kwargs):
        ...
        self.widget = forms.ClearableFileInput(attrs={'accept':kwargs.pop('accept', None)})
        super(ContentTypeRestrictedFileField, self).__init__(*args, **kwargs)

When we create instance with param accept=”.pdf,.txt”, in popup with file structure as a default we will see files with passed extension.

Just a minor tweak to @Thismatters answer since I can’t comment. According to the README of python-magic:

recommend using at least the first 2048 bytes, as less can produce incorrect identification

So changing 1024 bytes to 2048 to read the contents of the file and get the mime type base from that can give the most accurate result, hence:

def validate_extension(file):
    valid_mime_types = ["application/pdf", "image/jpeg", "image/png", "image/jpg"]
    file_mime_type = magic.from_buffer(file.read(2048), mime=True) #  Changed this to 1024 to 2048

    if file_mime_type not in valid_mime_types:
        raise ValidationError("Unsupported file type.")

    valid_file_extensions = [".pdf", ".jpeg", ".png", ".jpg"]
    ext = os.path.splitext(file.name)[1]

    if ext.lower() not in valid_file_extensions:
        raise ValidationError("Unacceptable file extension.")