File uploads but getting undefined JSON error
Hi Jayden, thanks for making this awesome package! I'm running into this weird issue, my actual file gets uploaded, but I get this error: 

_SyntaxError: Unexpected token u in JSON at position 0
    at JSON.parse (<anonymous>)
    at form.parse (/Users/vlady/WebProjects/yupty-api/node_modules/apollo-upload-server/src/index.js:20:31)
    at IncomingForm.<anonymous> (/Users/vlady/WebProjects/yupty-api/node_modules/formidable/lib/incoming_form.js:102:9)
    at emitOne (events.js:96:13)
    at IncomingForm.emit (events.js:188:7)
    at IncomingForm._error (/Users/vlady/WebProjects/yupty-api/node_modules/formidable/lib/incoming_form.js:290:8)
    at IncomingMessage.<anonymous> (/Users/vlady/WebProjects/yupty-api/node_modules/formidable/lib/incoming_form.js:120:12)
    at emitNone (events.js:86:13)
    at IncomingMessage.emit (events.js:185:7)
    at abortIncoming (_http_server.js:284:11)
    at Socket.serverSocketCloseListener (_http_server.js:297:5)
    at emitOne (events.js:101:20)
    at Socket.emit (events.js:188:7)
    at TCP._handle.close [as _onclose] (net.js:501:12)_

My server looks like this:
```javascript
app.use('/graphql',
	authenticate,
	bodyParser.json(),
	// Uploads
	apolloUploadExpress({
		// Optional, defaults to OS temp directory
		uploadDir: './uploads',
	}),
	graphqlExpress(req => {
		const query = req.query.query || req.body.query;
		if (query && query.length > 2000) {
			// Probably indicates someone trying to send an overly expensive query
			throw new Error('Query too large.');
		}

		return {
			schema: makeExecutableSchema({
				typeDefs: schema,
				resolvers,
			}),
			context: {
				user: req.user,
				token: req.headers.authorization,
			},
		};
	}),
);
```

I've been trying to fix and I have no idea what's going on...

The query on the client looks like this: 

```javascript

const CreateEventFormWithMutation = graphql(createEventMutation, {
	props: ({ ownProps, mutate }) => ({
		// This function will be available as a prop in the form:
		createEvent: ({
			venue,
			start_time,
			description,
			picture,
		}, artistIDs, genreIDs) => {
			const variables = {
				venue_id: venue.id,
				start_time,
				description,
				artistIDs,
				genreIDs,
			};

			// Add picture to variables if there is one
			if (Object.getOwnPropertyNames(picture).length > 0) {
				variables.picture = picture;
			}

			return mutate({
				variables,
				// Update event feed with data from form, no need for network request
				optimisticResponse: {
					__typename: 'Mutation',
					createEvent: {
						__typename: 'Event',
						// Note that we can access the props of the container at `ownProps`
						venue,
						start_time,
						artists: ownProps.newEvent.artists,
						description,
					},
				},
				// Name the query to update, in this case: the query from EventFeedContainer, "getEvents"
				updateQueries: {
					getEvents: (prev, { mutationResult }) => {
						return update(prev, {
							events: {
								$unshift: [mutationResult.data.createEvent],
							},
						});
					},
				},
			});
		},
	}),
})(CreateEventForm);

```