Support browser image transformations

[evan6seven]

From Issue #2808

Summary

Adds browser-side image transformations for media uploads, allowing projects to define generated image variants in CMS config and field config. Uploaded images can now be converted or resized before being persisted by the normal media upload flow.

What changed

• Adds an image_transformations config shape for defining generated variants.
• Supports root-level transformations and field-level overrides.
• Adds image conversion/resizing through Photon in the browser.
• Allows transformations to emit a selected default variant while optionally keeping or skipping the original upload.
• Handles both Immutable field maps and plain field objects when reading field transformation config.
• Uses byte-array image loading for Photon so browser File uploads work reliably.
• Keeps transformed-only draft uploads visible in image/file widgets by including generated _transformations/... files in draft media selection.
• Preserves generated transformation paths when inserting selected media into an entry field.

Why

Some sites need CMS uploads to produce optimized image assets, for example converting uploaded images to WebP or creating resized variants before the file is stored. This keeps that behavior inside the existing Decap media upload path instead of requiring a separate backend processing step.

The draft media changes are needed for image widgets when keep_original is disabled. In that case the only uploaded file may live under _transformations/<variant>/...; without including those generated paths in draft media selection, the upload succeeds but the widget cannot select or insert the generated asset.

Notes

• SVG uploads are not transformed.
• jpeg variant format is normalized to jpg for filenames and MIME handling.
• If no default variant is marked, the existing media selection behavior is preserved by ordering generated files predictably.
• Unit coverage was added for transformation config normalization, browser image loading, transformed upload persistence, transformed draft media selection, and public path insertion.

Testing

Manually end to end test with sample jpg files in an existing production Decap base project.

[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.