Support browser image transformations

[evan6seven]

From Issue #7107

Summary

Adds browser-side media processing for image uploads. Projects can process an uploaded image once before it is persisted by the normal media upload flow, with support for resizing, aspect-ratio cropping, metadata stripping through re-encoding, and optional JPEG/WebP conversion.

Configuration

media_processing:
  enabled: true
  format:
    enabled: true
    default: webp # jpeg | webp
  quality: 80 # 1-100
  strip_metadata: true
  width: null
  height: null
  aspect_ratio: 16_9

Notes on the config:

• media_processing can be set globally or on an individual image/file field; field config overrides the global config.
• format.enabled: true converts the upload to jpeg or webp.
• format.enabled: false keeps the uploaded image's original supported type (jpeg, png, or webp) while still allowing resize/crop/re-encode processing.
• quality is a plain number from 1 to 100; there is no auto quality mode.
• aspect_ratio accepts values like 16_9 or 16:9.
• SVG uploads are not processed.

Conversion methods

• Images are decoded and resized/cropped in the browser with Canvas.
• JPEG output uses Canvas toBlob with the configured quality.
• WebP output uses @jsquash/webp, so WebP compression works in browsers that cannot encode WebP through Canvas, including Safari.
• PNG is supported only as an original pass-through type when format conversion is disabled; it is not a configurable conversion target.
• WebAssembly assets from @jsquash/webp are emitted through the webpack WASM asset rule.

What changed

• Adds the media_processing config schema and TypeScript declarations.
• Reads media processing config from root config or field-level overrides.
• Runs a single processed output through the existing media persistence flow.
• Uses processed filenames for replacement checks and media insertion.
• Preserves the original filename when format conversion is disabled.
• Keeps draft media selection and public path handling compatible with processed uploads.

[martinjagodic]

@evan6seven this would be a great feature in Decap CMS, but before we merge it, we need to agree on what it would look like. Did you see issue #7107? There was a lot of debate around how to do it. I think it addresses a similar question, but in much more detail.

You added the Photon dependency, which could be a good fit, but I have some concerns. Most notably, it's maintained by a single person. The history is not in favor of the longevity of such libraries, so we have to be really cautious before we use it.

[evan6seven]

Thanks, I had not seen that issue discussion. I will read it and incorporate decisions from there.

[yanthomasdev]

Amazing work @evan6seven, I see your PR is a draft so I'm leaving just a few initial comments/suggestions for you to consider

[evan6seven]

Having looked at the original issue thread what I took away was:

• Use minimal libraries
• Support a few cropping and resizing operations

I'm minimizing libraries by using Canvas for jpeg and png images.
In terms of libraries, I'm using @jSquash/webp. This seems necessary as webp not supported in Safari's Canvas encoding. Do we want to add @jSquash/avif?

@yanthomasdev thanks for the feedback, I've updated based on it.