Fix problems with new forms

app/wtforms/validators.py

@@ -0,0 +1,14 @@
+from wtforms.validators import ValidationError
+
+
+def FileSize(max_size_mb):
+    max_size_b = max_size_mb * 1024 * 1024
+
+    def file_length_check(form, field):
+        if len(field.data.read()) >= max_size_b:
+            raise ValidationError(
+                f'File size must be less or equal than {max_size_mb} MB'
+            )
+        field.data.seek(0)
+
+    return file_length_check