Fix for editing an exercise after adding a thumbnail

[d0sadata]

Summary

Resolves #3158
The issue was a check order that wasn't executed properly, Misha helped me find it and solve it.
the order now first checks if the validation for file upload is required for exerice files or not.

Manual verification steps performed

1. Tested on local environment

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Reviewer guidance

Same as the issue reporting, create a new Exercise,
Add a thumbnail
Go to save

Contributor's Checklist

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Reviewer's Checklist

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[sairina]

@pcenov, this PR seems to fix the issue you described in #3158 ; however, I noticed that there's a follow-up problem (not necessarily related to this fix) where if a user then deletes the thumbnail and saves/clicks on "Finish", when the user is back on the channel editing page, the thumbnail that was deleted is still there. Would you mind checking to see if this is an issue, as well?

[sairina]

lgtm - this change fixes the reported issue.

[MisRob]

Thank you, @d0sadata