add voucher id to file upload method

[mrvnklm]

to add the possibility to upload files to exisiting vouchers by just adding the voucher id

file()->upload(string $filepath, string $type, ?string $voucherId = null)

[Sysix]

Hello @mrvnklm

thanks for the PR. 2 things are bugging me:

1. The Client refers to 2 Endpoints, We have already a Client/Voucher which should be the prefered Place
2. The $type Parameter is not required for the endpoint

I implemented a new method for the next major release: fbdbfdc