Note requirement to explicitly enabling multipart as an input format (#1943)

V3.3 has become more strict about input format checking (intentionally or otherwise) and users need to be more rigorous in allowing multipart when using it for file uploads.

Prior to v3.3, I suspect people should have been defining this, but it wasn't being checked, so many won't have done.

Not sure my "note" format is in the preferred document style - possibly we don't want to mention the version for example?  Feel free to edit or request changes.

Author: paullallier

core/file-upload.md

@@ -79,6 +79,7 @@ use Vich\UploaderBundle\Mapping\Annotation as Vich;
         new Get(),
         new GetCollection(),
         new Post(
+            inputFormats: ['multipart' => ['multipart/form-data']],
             controller: CreateMediaObjectAction::class, 
             deserialize: false, 
             validationContext: ['groups' => ['Default', 'media_object_create']], 
@@ -124,6 +125,7 @@ class MediaObject
     }
 }
 ```
+Note: From V3.3 onwards, `'multipart/form-data'` must either be including in the global API-Platform config, either in `formats` or `defaults->inputFormats`, or defined as an `inputFormats` parameter on an operation by operation basis.
 
 ### Creating the Controller