Validate that file doesn’t exist in subject

Author: quis

app/main/forms.py

@@ -3214,6 +3214,7 @@ def validate_report_has_been_processed(self, field):
 class TemplateEmailFilesUploadForm(StripWhitespaceForm):
     def __init__(self, *args, template, **kwargs):
         self.existing_file_names = InsensitiveSet(file.filename for file in template.existing_files)
+        self.placeholders_in_subject = InsensitiveSet(UtilsField(template.subject).placeholders)
         super().__init__(*args, **kwargs)
 
     allowed_file_formats = {
@@ -3253,6 +3254,12 @@ def validate_file(self, field):
         if field.data.filename in self.existing_file_names:
             raise ValidationError(f"Your template already has a file called ‘{field.data.filename}’")
 
+        if field.data.filename in self.placeholders_in_subject:
+            raise ValidationError(
+                f"You cannot put a file in the subject of a template "
+                "– remove ((field.data.filename)) or rename your file"
+            )
+
 
 class TemplateEmailFileLinkTextForm(StripWhitespaceForm):
     link_text = GovukTextInputField("Link text (optional)")