FileAttachment in a subdirectory is not resolved correctly

[mbostock]

For example, in docs/javascript/files.md:

```js
const gistemp = FileAttachment("gistemp.csv").csv({typed: true});
```

This should resolve to docs/javascript/gistemp.csv. However, the path is not rewritten in the client:

{"name": "gistemp.csv", "mimeType": "text/csv"}

and hence the client requests /_file/gistemp.csv which is a 404.