Client-Side Media: Move image_editor_output_format filtering to the upload response

[adamsilverstein]

Parent Issue

Part of #74333

Related trac ticket: https://core.trac.wordpress.org/ticket/64677

Context

In WordPress/wordpress-develop#10868 (Editor: PHP backports for Client Side Media), the image_editor_output_format filter is currently applied in the REST API index (get_index) by iterating over a list of possible input MIME types with an empty $output_formats array and no $filename parameter. As @westonruter pointed out, this doesn't match how wp_get_image_editor_output_format() normally applies the filter — it passes the actual filename and a pre-populated default mapping.

Problem

The current approach:

• Passes an empty array as the default value instead of the normal default mapping
• Does not supply a $filename parameter to the filter
• Re-filters the same $output_formats variable across iterations, accumulating results in a way that doesn't match the server-side behavior
• Is applied generically rather than per-upload, so plugins can't make decisions based on the actual input file

Proposed Solution

Move the image_editor_output_format logic from the REST API index response into the initial image upload response. This is already the pattern used for EXIF rotation data in client-side media processing.

Benefits:

• The actual input filename and MIME type will be available, so the filter operates just like it does on the server side
• Plugins that hook into image_editor_output_format will receive meaningful parameters
• Keeps per-file processing logic in the upload response where it belongs

References

• WordPress/wordpress-develop PR: #10868
• Review comment: Editor: PHP backports for Client Side Media wordpress-develop#10868 (comment)
• Response: Editor: PHP backports for Client Side Media wordpress-develop#10868 (comment)