Support file_input elements on modals

[ObedSj]

Description
Support file_input element

"blocks": [  
{
  "type": "input",
  "block_id": "input_block_id",
  "label": {
    "type": "plain_text",
    "text": "Upload Files"
  },
  "element": {
    "type": "file_input",
    "action_id": "file_input_action_id_1",
    "filetypes": ["jpg", "png"],
    "max_files": 5,
  },
}
]

(Optional) Slack's documentation
file_input

[parsley42]

@ObedSj I've just noticed you've started maintaining a separate fork of slack-go/slack, which is fine/cool - but, do you have any interest in being added as a maintainer for the main project?