A field type might be nice, but this is completely doable with the existing ACF fields and a filter or 2.
I would use a select field and https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/ using scandir http://php.net/manual/en/function.scandir.php to get the files to populate the field.
Then I would create a format value filter https://www.advancedcustomfields.com/resources/acf-format_value/ to convert the file name into a URL. But you could also set the value to the url when populating the choices.
