issue #2300 (not keeping values in register.tpl) -> still not working for checkboxes and file uploading

[nikto1]

1. You will get the error

` Notice: Undefined variable: register_custom_field_value in \www\opencart2\catalog\view\theme\default\template\account\register.tpl on line 141  `

for each checkbox.

2. If you make a custom field mandatory and for file uploading:
   Upload a file, then press the button and the field is still required.
   In other words: the check if the file was uploadad or not don't work.

Greetings