How use [rewrite_rules][conditions] with REQUEST_FILENAME in .ini?

### HHVM Version

3.18.1

### Standalone code, or other way to reproduce the problem

I expected to ignore only files, the folders would still go through rewrite (note that the ini is "commented"):

```
hhvm.virtual_host[default][check_existence_before_rewrite] = false

;hhvm.virtual_host[default][rewrite_rules][common][conditions][-d][pattern] = "%{REQUEST_FILENAME}"
;hhvm.virtual_host[default][rewrite_rules][common][conditions][-d][type] = request
;hhvm.virtual_host[default][rewrite_rules][common][conditions][-d][negate] = true

hhvm.virtual_host[default][rewrite_rules][common][conditions][-f][pattern] = "%{REQUEST_FILENAME}"
hhvm.virtual_host[default][rewrite_rules][common][conditions][-f][type] = request
hhvm.virtual_host[default][rewrite_rules][common][conditions][-f][negate] = true

hhvm.virtual_host[default][rewrite_rules][common][pattern] = "(.*)"
hhvm.virtual_host[default][rewrite_rules][common][to] = "index.php"
hhvm.virtual_host[default][rewrite_rules][common][qsa] = true
```

> This is an "illustrative" code, I believe that `-d` and `-f` flags aren't supported.

### Expected result

Ignore rewrite_rules from form pattern

---

I reviewed the documentation, but only found the "check_existence_before_rewrite", Is there any way to check for the existence of **specific files** using proxygen before applying rewrite?