Can’t install if wp-content/uploads in not empty

[JalilArfaoui]

Trying to run composer install on an existing project gives error and fails :

In Filesystem.php line 314:
Could not delete wordpress/wp-content/uploads: 
Error: composer.phar install failed. Please check the lines above to find out why

I would expect that an existing uploads folder should be OK and not prevent the whole app from installing …