title: Collection of custom type not saving files in database EasyAdmin
body: <p>If have a collection of a custom type (PhotoType, which is basically just a FileType with an Image constraint), like this:</p>
  <pre><code>CollectionField::new('photos')
              -&gt;setFormTypeOptions([
                  'entry_type' =&gt; PhotoType::class,
              ])
              -&gt;hideOnIndex(),
  </code></pre>
  <p>It displays well the PhotoType with a &quot;add button&quot; which allows us to create a new one, but if I choose a file and save the form, the number of Photos are well saved, but the arrays are empty. No filename is saved. Like this:</p>
  <pre><code>a:2:{i:1;a:0:{}i:2;a:0:{}}
  </code></pre>
  <p>I'm using EasyAdmin version 3.5.17</p>
  <p>My PhotoType looks like this:</p>
  <pre><code>class PhotoType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options): void
      {
          $builder
              -&gt;add('photo', FileType::class, [
                  'mapped' =&gt; false,
                  'required' =&gt; false,
                  'constraints' =&gt; [
                      new Image([
                          'maxSize' =&gt; '10M',
                          'mimeTypesMessage' =&gt; 'Please upload a valid image document',
                      ])
                  ],
                  'label' =&gt; 'Choose a file'
              ])
          ;
      }
  
      public function configureOptions(OptionsResolver $resolver): void
      {
          $resolver-&gt;setDefaults([
          ]);
      }
  }
  </code></pre>