How to parse a form but not automatically in case a form element is missing

I have the following function:

```
app.post("/upload", upload.single('file'), function (request, response, next) {

if (request.file==null) {
response.json("No file selected.");
return;
}
}
```

If the form does not have the file submitted does the call upload.single("file") cause errors? I've included a check to see if the form property exists.

Is there an alternative way to parse the form body?

Can I do something like this?
```
app.post("/upload", function (request, response, next) {
var test = upload.single('file');
if (request.file==null) {
response.json("No file selected.");
return;
}
}
```

Start at the app.post('/upload', upload.single('file'), ...) entry point and inspect how upload.single('file') populates request.file when the multipart form omits the file. Confirm the documented behavior and identify whether an alternative form-body parsing approach is supported; done means providing a clear usage answer for both cases.

Written by the indexing model from the issue text.