Add WebP support and dedicated filter for company logo uploads

[masteradhoc]

Fixes #2932

Changes Proposed in this Pull Request

• Added webp as a default allowed mime type for company logo uploads
• Added job_manager_company_logo_allowed_mime_types filter so developers can customize allowed mime types for company logos without affecting other file fields
• Applied the same filter to both the form validation path and the Ajax uploader path (job_manager_get_allowed_mime_types()) so they stay in sync
• Fixed the frontend Ajax upload preview not showing an image thumbnail for .webp files

Testing Instructions

• Submit a job listing and upload a .webp file as the company logo — it should upload successfully and show an image preview
• Confirm non-image file types still trigger the correct error
• Add the filter below and confirm a custom type (e.g. SVG) is accepted in both the drag-and-drop Ajax uploader and on form validation:

add_filter( 'job_manager_company_logo_allowed_mime_types', function( $types ) {
    $types['svg'] = 'image/svg+xml';
    return $types;
} );

Release Notes

• Company logo uploads now accept WebP images by default
• New filter job_manager_company_logo_allowed_mime_types allows customizing allowed file types for the company logo field

New or Updated Hooks and Templates

• Filter: job_manager_company_logo_allowed_mime_types — Filters the allowed mime types for the company logo upload field. Applies to both the form validation and the Ajax file uploader. Receives an array of extension => mime_type pairs. Note: the form validation path uses plain extension keys (e.g. 'webp'), while the Ajax uploader uses pipe-separated keys (e.g. 'jpg|jpeg|jpe') — both are passed through this same filter.

Deprecated Code

Screenshot / Video