fixed file property accessibility

[philipphoffmann]

The file property should be private and there should be a setter for the property (so the form can save it).
Otherwise doctrine complains about the public property: Field 'file' in class 'Acme\DemoBundle\Entity\Document' must be private or protected. Public fields may break lazy-loading.

[stof]

This is not needed here as this property is not mapped by Doctrine.

[philipphoffmann]

Yeah I know, but Doctrine is complaining about the fact that the property is public. So should this be fixed in Doctrine or in the example?

[stof]

I think this is a bug in the Doctrine tool validating this.

[yahuarkuntur]

Any updates on this issue?

[weaverryan]

I'm going to close this PR for now for 2 reasons:

1. This entire document is going to be heavily updated (see Refactor the Uploading Document #2346).
2. When we do that, I think we should use protected/private properties anyways since it's a better practice (even if it makes the docs a bit longer). So, that should fix this one way or another.

Even if the bug is in Doctrine, we should code to make sure things work. But the rewrite of this section will take care of this - there are other things (see the above issue) that I dislike about this entry :).

Thanks!