Overall this looks great, I will give this a test specifically to verify that the filter works to set output to a single mime type (eg transform jpeg->webp) and also test with and without WebP support.

Can we add some additional tests that validates new functionality as well?

Overall this looks great, I will give this a test specifically to verify that the filter works to set output to a single mime type (eg transform jpeg->webp) and also test with and without WebP support.

Can we add some additional tests that validates new functionality as well?

@adamsilverstein @felixarntz @mitogh I have found and fixed a few issues while working on phpunit tests. Could you, please, take another look at the implementation?

Also I have refactored a bit our phpunit tests for the webp-uploads module to make it easier to check sources. Let me know what you think.

I did some testing to verify that the filter mapping works as expected, using the filter to test the following scenarios when uploading both jpegs and webp images:

• generate only WebP sub-sizes
• generate only jpeg sub-sizes
• generate both WebP and jpeg sub-sizes
• generate both, reversing the default

In code:

add_filter( 'webp_uploads_upload_image_mime_transforms', function() {
	$test = 'only-webp'; // Try each

	switch ( $test ) {
		case 'only-webp':
			return [
				'image/jpeg' => [ 'image/webp' ],
				'image/webp' => [ 'image/webp' ],
			];
		case 'only-jpg':
			return [
				'image/jpeg' => [ 'image/jpeg' ],
				'image/webp' => [ 'image/jpeg' ],
			];
		case 'both':
			return [
				'image/jpeg' =>  [ 'image/jpeg', 'image/webp' ],
				'image/webp' =>  [ 'image/webp', 'image/jpeg' ],
			];
		case 'reverse':
			return [
				'image/jpeg' =>  [ 'image/webp', 'image/jpeg' ],
				'image/webp' =>  [ 'image/jpeg', 'image/webp' ],
			];
		case 'empty':
		default:
			return [];
	}

});

I reviewed the files generated as well as the meta data. Comments below:

✅ only-webp: worked as expected, only webp images generated for either webp or jpeg uploads
⚠️ only-jpeg: worked for jpegs, only jpeg was generated. when I uploaded a WebP it generated webp, I was expecting jpeg - not sure of the use case, but was expecting this to work
✅ both: worked as expected, both formats created including -scaled version, worked for both jpeg and webp uploads
❌ reverse: didn't work as expected - behavior was the same as 'both', I was expecting the alternate mime type to be stored as the "primary" image, meaning stored in the sizes array, and first in the sources arrays. Not sure this is an issue or if others will expect this behavior as well. Not sure this is completely needed here, this is a little trickier to achieve in the plugin, for the core patch it will be much easier to just start with the first mime type.
✅ 'empty': returning an empty array worked as expected, essentially disabling any additional mime type generation. When I uploaded a WebP only WebP sub-sizes were generated, when I uploaded a JPEG, only JPEG sub-sizes were created.

⚠️ only-jpeg: worked for jpegs, only jpeg was generated. when I uploaded a WebP it generated webp, I was expecting jpeg - not sure of the use case, but was expecting this to work

Hm... it works correctly on my end. When I upload a webp image, all subsizes have been created in the jpeg format. The original image is still in webp though. Is it what you mean, @adamsilverstein?

❌ reverse: didn't work as expected - behavior was the same as 'both', I was expecting the alternate mime type to be stored as the "primary" image, meaning stored in the sizes array, and first in the sources arrays. Not sure this is an issue or if others will expect this behavior as well. Not sure this is completely needed here, this is a little trickier to achieve in the plugin, for the core patch it will be much easier to just start with the first mime type.

@adamsilverstein see #187 (comment):

If the filter is being used in a way so that for a given upload's original MIME type that same MIME type is not part of the transformation array (e.g. a developer decides that no JPEGs should be generated even if a JPEG is uploaded), the first format in the transformation list should be used as the default format to generate the image sizes for. In order to force WordPress core to not generate the original MIME type, the image_editor_output_format needs to be used. A callback should be added to that which calls the webp_uploads_get_supported_image_mime_transforms() function firing the filter and if the original format is not in the list, the filter value should be modified to generate the appropriate other image format. For that purpose and this particular scenario, the original implementation of the WebP Uploads module (see #32) should essentially be reintroduced.

Basically, what it says is that we need to select the alternate mime type for the "primary" image only if the original mime type is not included into the transformation list which is not the case in your situation. Does it make sense to you or I miss something?