Changed 'object' return type to 'array'

Signed-off-by: Elliot Condon <e@elliotcondon.com>
Added v4 compatibility return_type of 'object'

https://github.com/AdvancedCustomFields/acf5-beta/issues/71
Signed-off-by: Elliot Condon <e@elliotcondon.com>
Added image / file upgrade support for new return_type setting

https://github.com/AdvancedCustomFields/acf5-beta/issues/71
Signed-off-by: Elliot Condon <e@elliotcondon.com>