Fixes https://linear.app/chatwoot/issue/CW-5752/fix-nomethoderror-when-processing-video-files-in-slack-integration
#### Problem
When users shared video files (like MP4) through Slack, the `file_type`
method in `SlackMessageHelper` would return `nil` for unsupported file
types. This caused a `NoMethodError (undefined method 'to_sym' for nil)`
when the attachment was being processed, as the system expected a symbol
value for the `file_type` attribute.
#### Solution
- Added video file type support in the `file_type` method case statement
- Added `else` clause to default unknown file types to `:file` instead
of returning `nil`
- This ensures `file_type` always returns a symbol, preventing the
`to_sym` error
d8da1f5bf3