[Validator] Ability to validate only uploaded files for Assert/File

| Q	| A | 
| ---	| --- | 
| New feature? | no | 
| BC breaks? | no |  
| Deprecations? | no | 
| Tests pass? | yes | 
| Fixed tickets | N/A | 
| License | MIT | 
| Doc PR | no | 

We use gaufrette filesystem mapped via SFTP and located very weird perfomance issue. We have such a code in our doctrine entities 
```
     * @Assert\File(
     *     maxSize="100M",
     *     mimeTypes={"image/png", "image/jpeg", "image/jpg", "image/bmp", "image/gif"},
     * )
     public $file;
```
In background it check file each time we load entity. If we load 100 entities - validator do lot of the useless requests to remote server and page load speed decrease.

I have created small patch for this issue, what do you think about it?