While #28450 overhauled a ton of forms work, the support for this custom file input was ineffective. This updates the selector to target the custom field and gray out the entire field.
Fixes#28176.
* Initial spike of consolidated form checks
* Stub out forms rearrangement
- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS
* Move input group Sass file to forms subdir
* Start to split and move the docs around
* Simpler imports
* Copyediting
* delete overview file
* Remove commented out code
* remove the custom-forms import
* rewrite flex-check as form-check, replace all custom properties
* Remove old forms doc
* stub out new subpage link section
* update migration guide
* Update nav, forms overview in page nav, and descriptions
* fix check bg position
* fix margin-top calculation
* rename .custom-select to .form-select
* Update validation styles for new checks
* add some vertical margin, fix inline checks
* fix docs examples
* better way to do this contents stuff, redo the toc while i'm at it
* page restyle for docs while here
* un-callout that, edit text
* redo padding on toc
* fix toc
* start to cleanup checks docs
* Rewrite Markdown tables into HTML
* Redesign tables, redo their docs
* Replace Open Iconic icons with custom Bootstrap icons
* Redesign the docs navbar, add a subheader, redo the sidebar
* Redesign docs homepage a bit
* Simplify table style overrides for docs tables
* Simplify docs typography for page titles and reading line length
* Stub out icons page
* Part of sidebar update, remove migration from nav.yml
* Move toc CSS to separate partial
* Change appearance of overview page
* fix sidebar arrow direction
* Add footer to docs layout
* Update descriptions
* Drop the .form-group class for margin utilities
* Remove lingering form-group-margin-bottom var
* improve footer spacing
* add headings to range page
* uncomment form range css
* Rename .custom-range to .form-range
* Drop unused docs var
* Uncomment the comment
* Remove unused variable
* Fix radio image sizing
* Reboot update: reset horizontal ul and ol padding
* de-dupe IDs
* tweak toc styles
* nvm, fix dropdown versions stuff
* remove sidebar nav toggle for now
* broken html
* fix more broken html, move css
* scss linting
* comment out broken helper docs
* scope styles
* scope styles
* Fixes#25540 and fixes#26407 for v5 only
* Update sidebar once more
* Match new sidenav order
* fix syntax error
* Rename custom-file to form-file, update paths, update migration docs for previous changes in #28696
* rename back
* fix size and alignment
* rename that back too
