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)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

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[codecov]

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 ⚠️
...m/slack/api/model/block/element/BlockElements.java	0.00%	1 Missing ⚠️
...m/slack/api/util/json/GsonBlockElementFactory.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             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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