You can use simple form validation, as everywhere in Django.
Pseudocode below
admin.py
class YourModelAdminForm(forms.ModelForm):
def clean_your_field(self):
if format_is_not_valid:
raise forms.ValidationError('Format is not valid')
class YourModelAdmin(admin.ModelAdmin):
form = YourModelAdminForm
admin.site.register(YourModel, YourModelAdmin)
or you can create your custom validator and use it on model field
models.py
class YourModel(models.Model):
your_field = models.FileField(validators=[your_validator])