Fix bug multiple choice with video. BLD-788.

**Jira issue**
[BLD-788](https://edx-wiki.atlassian.net/browse/BLD-788)

**Description**
Multiple choice/checkbox problems in Studio are marked "incorrect" if below video.

**Problem**
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` on the page, the page-wide selector returned the Video's file input, and the `check_fd()` method went on believing that the `multiple choice` had a file input.

**Solution**
The correct way is to look for file inputs only in the current element.

**Reviewers**
- @polesye