fix: file input handler

[leangseu-edx]

TL;DR - hasFile run only once, it will always be null

JIRA: https://2u-internal.atlassian.net/browse/CR-5738

Developer Checklist

• [ ] Test suites passing
• [ ] Documentation and test plan updated, if applicable
• [ ] Received code-owner approving review
• [ ] Bumped version number package.json

Testing Instructions

[ How should a reviewer test this PR? ]

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• [ ] I've done a visual code review
• [ ] I've tested the new functionality

FYI: @openedx/content-aurora

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (541a661) 100.00% compared to head (7391ef2) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #323 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 122 122 Lines 1269 1268 -1 Branches 251 249 -2 ========================================= - Hits 1269 1268 -1 ``` | [Impacted Files](https://codecov.io/gh/openedx/frontend-app-gradebook/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx) | Coverage Δ | | |---|---|---| | [.../components/GradesView/ImportGradesButton/hooks.js](https://codecov.io/gh/openedx/frontend-app-gradebook/pull/323?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openedx#diff-c3JjL2NvbXBvbmVudHMvR3JhZGVzVmlldy9JbXBvcnRHcmFkZXNCdXR0b24vaG9va3MuanM=) | `100.00% <100.00%> (ø)` | |

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