Wrong file name in file type when uploading

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Hey,

``` javascript
nga.field('image', 'file').uploadInformation({
'url': imageUrl,
'headers': {'Authorization': token},
'apifilename': 'image'
}),
```

In the request the name property has a wrong value :

```
Content-Disposition: form-data; name="file"; filename="toto.jpg"
```

It should be "image".
The html markup of the input has the correct value:

``` html

```

I can't find the problem, ng-admin seems to correctly pass the filename to ng-file-upload.

Reproduce the upload using the JavaScript snippet and compare the input's name="image" with the multipart request's name="file". Trace how ng-admin passes apifilename to ng-file-upload, then verify that the request uses image while retaining the uploaded filename.

Written by the indexing model from the issue text.