Adding mp4 files support for NFTs

[vbaranov]

#3392

Motivation

Add mp4 files support for NFTs

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build d475b247938c8f818eeba66cd2f773c417e34801-PR-3406

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 23 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+1.3%) to 67.49%

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Files with Coverage Reduction	New Missed Lines	%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%
lib/block_scout_web/channels/address_channel.ex	2	36.96%
lib/block_scout_web/notifier.ex	2	82.95%
lib/block_scout_web/views/tokens/instance/overview_view.ex	18	23.81%

Totals
Change from base Build b21659f0667e637f20a1439ab2938f9ad21342e3:	1.3%
Covered Lines:	2049
Relevant Lines:	3036

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💛 - Coveralls