File Upload: Correctly instruct how to handle updates

[weaverryan]

Currently, if you follow the file upload cookbook, you will not be adequately handling "updates" for two reasons:

1. If you only modify the file property, then the doctrine update listeners are not triggered
2. If you do change the uploaded file, the old file is not deleted

Related is this commit, whose caution can hopefully be removed at some point: 808da21. Also, see #564.

[oscargala]

Today I came across the problem that the preupdate callback isn't triggered when you just change the $file attribute. Has there been any progress on this issue?

[Sgoettschkes]

In the cookbook there is now a note stating that problem and also offering one workaround: http://symfony.com/doc/current/cookbook/doctrine/file_uploads.html#using-lifecycle-callbacks

Is this enough or is there a need for more detailed information? If so, what would be the best way (creating an advanced example with the way described in symfony/symfony#5150 or an extended note or maybe something else)?

[wouterj]

closing this in favor of #2346, it's mentioned there so we don't loose the content of this one