How does one use magic to verify file type in a Django form clean method?

Question

I have written an email form class in Django with a FileField. I want to check the uploaded file for its type via checking its mimetype. Subsequently, I want to limit file t

Answer 1

In case you're handling a file upload and concerned only about images, Django will set content_type for you (or rather for itself?):

from django.forms import ModelForm
from django.core.files import File
from django.db import models
class MyPhoto(models.Model):
    photo = models.ImageField(upload_to=photo_upload_to, max_length=1000)
class MyForm(ModelForm):
    class Meta:
        model = MyPhoto
        fields = ['photo']
photo = MyPhoto.objects.first()
photo = File(open('1.jpeg', 'rb'))
form = MyForm(files={'photo': photo})
if form.is_valid():
    print(form.instance.photo.file.content_type)

It doesn't rely on content type provided by the user. But django.db.models.fields.files.FieldFile.file is an undocumented property.

Actually, initially content_type is set from the request, but when the form gets validated, the value is updated.

Regarding non-images, doing request.FILES['name'].read() seems okay to me. First, that's what Django does. Second, files larger than 2.5 Mb by default are stored on a disk. So let me point you at the other answer here.

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For the curious, here's the stack trace that leads to updating content_type:

django.forms.forms.BaseForm.is_valid: self.errors
django.forms.forms.BaseForm.errors: self.full_clean()
django.forms.forms.BaseForm.full_clean: self._clean_fields()
django.forms.forms.BaseForm._clean_fiels: field.clean()
django.forms.fields.FileField.clean: super().clean()
django.forms.fields.Field.clean: self.to_python()
django.forms.fields.ImageField.to_python