Webui/file upload (#18694)
* webui: fix restrictive file type validation
* webui: simplify file processing logic
* chore: update webui build output
* webui: remove file picker extension whitelist (1/2)
* webui: remove file picker extension whitelist (2/2)
* chore: update webui build output
* refactor: Cleanup
* chore: update webui build output
* fix: update ChatForm storybook test after removing accept attribute
* chore: update webui build output
* refactor: more cleanup
* chore: update webui build output
