Fix bug multiple choice with video. BLD-788.

The Video element contains an input with type file. When the
Check button was clicked for the multiple choice problem, the
code was acting in different ways when there as a Video on the
page and when there wasn't. This is because in the check_fd()
method there was a selector that was looking for inputs of type
file on the whole page instead of just in the current problem.
When there was a Video pn the page, the page-wide selector
returned the Video's file input, and the check_fd() method
went on believing that the multiple choice has a file input.

The correct way is to look for file inputs only in the current
element.