Custom attributes can be added through the plugin filter. This option is meant to be a helper that adds a class with the name and value of the attribute. An advantage of using this property rather than using the blocks.registerBlockType filter is that it should work if you need to deprecate the attribute.
Keep in mind that it will simply add a class in the .mbc-container div, and that a string, number or boolean value can be used.
string or number
If the attribute is a string or a number the class will include the name and the value example_attribute_name => .mbc-example_attribute_name-the_value.
boolean
If the attribute is a boolean the class will include the name and enabled or disabled example_attribute_name => .mbc-example_attribute_name-enabled.