If you upload a csv file (see MDL-41347 testing instructions for details) that includes the path to a file that isnt actually a Moodle backup the preview tells you everything is fine. Then when you continue you get the following fatal error.

error/unable_to_find_conversion_path

More information about this error
Debug info:
Error code: unable_to_find_conversion_path
$a contents:
Stack trace:

line 436 of /backup/controller/restore_controller.class.php: restore_controller_exception thrown
line 730 of /admin/tool/uploadcourse/classes/course.php: call to restore_controller->convert()
line 209 of /admin/tool/uploadcourse/classes/processor.php: call to tool_uploadcourse_course->proceed()
line 97 of /admin/tool/uploadcourse/index.php: call to tool_uploadcourse_processor->execute()

This happened because I gave the path to a tgz file that was similarly named to a mbz file.