Fix #1235 Add file input block element support

[seratch]

This pull request resolves #1235

Category (place an x in each of the [ ])

• [ ] bolt (Bolt for Java)
• [ ] bolt-{sub modules} (Bolt for Java - optional modules)
• [ ] slack-api-client (Slack API Clients)
• [x] slack-api-model (Slack API Data Models)
• [ ] *slack-api--kotlin-extension** (Kotlin Extensions for Slack API Clients)
• [x] slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (98b6a77) 74.04% compared to head (b811047) 74.05%.

Files	Patch %	Lines
...ck/element/container/MultiBlockElementContainer.kt	0.00%	2 Missing :warning:
...m/slack/api/model/block/element/BlockElements.java	0.00%	1 Missing :warning:
...m/slack/api/util/json/GsonBlockElementFactory.java	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1236 +/- ## ============================================ + Coverage 74.04% 74.05% +0.01% - Complexity 4083 4090 +7 ============================================ Files 440 441 +1 Lines 12995 13016 +21 Branches 1310 1310 ============================================ + Hits 9622 9639 +17 - Misses 2606 2610 +4 Partials 767 767 ```

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