file upload

```r
bx_body_upload(path, folder_id = bx_remote(), name = NULL)
```

maybe something like:

```r
bx_request("files/content") %>%
  bx_body_upload(bx_local("numbers.csv")) %>%
  bx_req_run()
```

this suggests functions:

```r
bx_remote() # gets "default" folder_id (starts at `"0"`)
bx_remote_set(folder_id)
```