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

[adamsilverstein]

In ​#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 noted in ​this review comment, 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

• PR: ​#10868
• Review: ​@westonruter's comment
• Response: ​@adamsilverstein's response

[adamsilverstein]

Note: this probably needs fixing in GB and core. I will probably start in GB then backport to core.

[adamsilverstein]

I created a matching Gutenberg issue which I will work on first, then backport the changes to core. ​https://github.com/WordPress/gutenberg/issues/75784

[swissspidy]

This was added to the index so that the desired output mapping is known client-side. Iif a plugin wants to convert all JPEGs to WebP for example, the client-side media handling can do it automatically.

Of course it's not possible to pass a filename in that case. Updating documentation accordingly is reasonable IMO.

Move the image_editor_output_format logic from the REST API index response into the initial image upload response

But then the image is already uploaded and you can't convert it in the browser anymore.

[suhan2411]

I can reproduce the issue locally and would like to investigate moving the image_editor_output_format filtering earlier so it is reflected correctly in the upload response.

Working on a patch.