Ensure the -scaled image remains in the original uploaded format

[adamsilverstein]

Summary

When uploading very large images (over the big_image_size_threshold filter threshold), WordPress creates a scaled version of the image with the word-scaled appended to the file name. It stores a reference to the original image in the image meta original_image field, see this make post for more details.

WordPress subsequently uses the "scaled" image as if it were the original in most use cases (only going back to the true original image when editing the image). This causes the mime type shown in the media library for a large jpeg upload to show as image/webp because the scaled image is a WebP.

To fix that, in this PR I skip the -scaled image, leaving it in the original format of the uploaded image.

This matches the behavior of uploading a smaller image that isn't over the big_image_size_threshold, improving consistency.

Fixes #67

Relevant technical choices

• Makes sense that the scaled image, which WordPress largely treats as the "original" would remain in the mime type of the upload
• Ensure the behavior is consistent between large and smaller image uploads.

Checklist

• PR has either [Focus] or Infrastructure label.
• PR has a [Type] label.
• PR has a milestone or the no milestone label.

[felixarntz]

Looks good, just two comments.

[mitogh]

Additionally if we are going with this route once this is merged we should be able to close:

• #122

[adamsilverstein]

Additionally if we are going with this route once this is merged we should be able to close:

• *.jpg is not generated for scaled full size image. #122

Thinking we should generate both, see #142 (comment)