support file url in $network.upload

Nobody has claimed this yet.

Currently $network.upload uses the mostly undocumented `data` property to upload a data uri to S3.
Instead, as discussed with @gliechtenstein on slack, it would be better to support using a local file url as the source of the data to be uploaded, which avoids having to hold the whole file being uploaded (which potentially can be very large) in app memory.
The syntax that will be used for the property is:
```
"data": {
"file_url": "..."
}
```

Start by locating the $network.upload entry point and the code that sends upload data to S3. Verify how the existing data property handles data URIs, then determine where the file_url form should be accepted and how completion can be tested without loading the whole file into app memory.

Written by the indexing model from the issue text.