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
* fix: downcase email when finding
* feat: add `from_email` class
* refactor: use `from_email`
* feat: add rule to disallow find_by email directly
* chore: remove redundant test
Since the previous imlpmentation didn't do a case-insentive search, a new user would be created, and the error would be raised at the DB layer. With the new changes, this test case is redundant
* refactor: use from_email
