Media: Update post mime type when image format is converted during upload

[adamsilverstein]

Summary

• Updates _wp_image_meta_replace_original() to also update the attachment's post_mime_type when the image is converted to a different format (e.g., HEIC to JPEG). Previously, the actual files on disk were correctly converted but the post's mime type remained unchanged.
• Fixes the REST API index to use wp_get_image_editor_output_format() when building image_output_formats, so the default HEIC-to-JPEG mapping is properly exposed to client-side media processing.

Trac ticket

https://core.trac.wordpress.org/ticket/XXXXX (to be created)

Test plan

• Upload a HEIC file via the media library (not using client-side media processing in the block editor) on a server with HEIC support (e.g., Imagick with HEIC support)
• Verify the attachment's mime type is image/jpeg after upload, not image/heic
• Verify the generated sub-sizes are JPEG files
• Verify the REST API index (/wp-json/) includes image/heic: image/jpeg in image_output_formats when client-side media processing is enabled
• Upload a regular JPEG/PNG to confirm no regressions in the normal upload flow

🤖 Generated with Claude Code

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