Initial

backend/app/form/file_with_img_preview.py

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+from django.conf import settings
+# from django.contrib.admin.widgets import AdminFileWidget
+# from django.core.exceptions import ValidationError
+from django.db import models
+from django.forms import ValidationError
+from django.forms.widgets import ClearableFileInput
+
+# class ImageFileWidget(AdminFileWidget):
+
+
+class ImageFileWidget(ClearableFileInput):
+    template_name = 'forms/img-file.html'
+
+    def get_context(self, name, value, attrs):
+        context = super().get_context(name, value, attrs)
+        context['MEDIA_URL'] = settings.MEDIA_URL
+        return context
+
+
+class FileWithImagePreview(models.FileField):
+    def formfield(self, **kwargs):
+        # if 'widget' not in kwargs:  # only if no other is set (admin UI)
+        kwargs['widget'] = ImageFileWidget
+        return super().formfield(**kwargs)
+
+
+class ImageValidator:
+    help_text = 'Ideal: 250 x 320 px (JPEG oder PNG)'
+
+    @staticmethod
+    def validate(value: 'models.FieldFile') -> None:
+        # TODO: make configurable
+        if value.size > 512 * 1024:
+            raise ValidationError('Datei darf maximal 512 KB groß sein.')
+
+        content_type = getattr(value.file, 'content_type', None)
+        if content_type not in ['image/png', 'image/jpeg', None]:
+            raise ValidationError('Nur JPEG und PNG Bilder werden unterstützt')