Stories attachments don't support SVG uploads

[andersan]

Minor issue, not likely to matter for actual users. Don't know if other file types are also not supported.

Steps to reproduce:

1. edit a story
2. upload an SVG
3. press the update button
4. try to edit the story again, and see below message

Error message:

Afterwards, I can't access the story to edit it at all.

[rudokemper]

Thanks @andersan. We're moving this to discussion for now but will revisit after some refactor work which will also hopefully solve this.