forms: allow prefilling of file fields (#46929)

tests/form_pages/test_file_field.py

@@ -396,3 +396,26 @@ def test_form_file_field_with_wrong_value(pub):
     assert logged_error.summary == 'Failed to set value on field "file"'
     assert logged_error.exception_class == 'AttributeError'
     assert logged_error.exception_message == "'str' object has no attribute 'time'"
+
+
+def test_form_file_field_prefill(pub):
+    FormDef.wipe()
+    formdef = FormDef()
+    formdef.name = 'test'
+    formdef.fields = [
+        fields.FileField(
+            id='0',
+            label='file',
+            prefill={'type': 'string', 'value': '{{ "test"|qrcode }}'},
+        )
+    ]
+    formdef.store()
+
+    resp = get_app(pub).get('/test/')
+    assert resp.form['f0$token']
+    assert resp.click('qrcode.png').content_type == 'image/png'
+    resp = resp.form.submit('submit')  # -> validation
+    resp = resp.form.submit('submit')  # -> submit
+    formdata = formdef.data_class().select()[0]
+    assert formdata.data['0'].base_filename == 'qrcode.png'
+    assert formdata.data['0'].get_content().startswith(b'\x89PNG')

wcs/fields.py

@@ -1672,6 +1672,9 @@ class FileField(WidgetField):
     def get_view_short_value(self, value, max_len=30, **kwargs):
         return self.get_view_value(value, include_image_thumbnail=False, max_len=max_len, **kwargs)
 
+    def get_prefill_value(self, user=None, force_string=True):
+        return super().get_prefill_value(user=user, force_string=False)
+
     def get_download_query_string(self, **kwargs):
         if kwargs.get('file_value'):
             return 'hash=%s' % kwargs.get('file_value').file_digest()