[FEATURE] Add accept field variable support to File fields for MIME type filtering

Problem

Currently, File fields do not respect the accept field variable for MIME type filtering, even though:
The UI allows users to set the accept field variable on File fields
Binary fields already support this feature
Image fields have built-in MIME type filtering (image/*)
This creates an inconsistent user experience and prevents content editors from restricting file types in File fields (e.g., limiting to video/mp4 for video uploads).

Goal

• Add accept field variable support to File fields to enable MIME type filtering, providing:
• Feature parity between Binary and File field types
• Consistent user experience across field types
• Better content governance by allowing administrators to restrict file types per field
• Alignment with user expectations - if the UI allows setting the variable, it should work
• Users will be able to restrict File fields to specific MIME types (e.g., video/mp4, application/pdf, video/*) just like Binary fields, while maintaining the ability to select files from the dotCMS server (which Binary fields don't support).

Target Personas

• Developer teams
• Content teams
• DevOps teams
• System administrators (dotCMS)

Demo Expectations

Setting up a File field with MIME type restriction

Acceptance Criteria

• Fields should process the accept field variable similar to Binary fields
• File selection dialogs and upload components should filter files based on the accept MIME types
• This should work for both file uploads and "select existing file" browser dialogs

User Stories

No response

Links

https://helpdesk.dotcms.com/a/tickets/35126

Start by comparing how Binary fields process the accept field variable, then trace the File field upload and existing-file selection dialogs described in the acceptance criteria. Done means both upload and server-file selection filter files by the configured MIME types while preserving existing File field behavior.