OpenRaster MIME Type

What does image/openraster mean?

The image/openraster media type identifies an OpenRaster document, normally stored with the .ora extension. OpenRaster is an interchange format for layered raster artwork. It gives painting and image-editing applications a shared alternative to using one application's native project format.¹

An .ora file is a ZIP archive, not a single bitmap. Its stack.xml file describes the canvas, nested layer groups, layer order, visibility, opacity, offsets, and compositing operations. Raster layers are normally stored as separate PNG files. The format can also reference SVG layer data.²³

Every conforming archive includes these important entries:²

The OpenRaster specification defines the media type inside the container, but image/openraster does not appear in IANA's current media-type registry.⁴ In practice, applications still use this identifier and the .ora extension consistently.

Why OpenRaster was created

Boudewijn Rempt and Cyrille Berger proposed OpenRaster in 2006 as a way to exchange layered images between editors.¹ The format deliberately borrows the transparent, ZIP-and-XML structure used by OpenDocument. Its current published specification is version 0.0.6.¹

The design has two complementary goals. A full-baseline reader can load and edit the layer stack, while a viewing-baseline reader can show the required mergedimage.png even if it cannot reproduce every editable feature.³ That flattened composite is a useful compatibility fallback, but it does not turn a basic image viewer into a layered editor.

OpenRaster remains an interchange format rather than an ideal final-delivery format. Krita's documentation notes that separate PNG layers can make .ora files relatively large and unsuitable for ordinary web publishing.⁵

How to open an .ora file

Use a graphics application with explicit OpenRaster support:

• Krita opens and saves .ora as a layered interchange document.⁵
• GIMP supports both OpenRaster import and export.⁶
• MyPaint and Scribus are also listed by the OpenRaster project as supporting applications.¹

Current browsers do not natively display OpenRaster documents. MDN's browser image guide covers the formats browsers decode and does not include OpenRaster.⁷ Serving an .ora file from a website will generally result in a download rather than an inline layered image.

Support in Poly

Poly recognizes image/openraster as an image type and maps .ora filenames to it. That recognition is visible in the shared MIME schema and the client MIME map. Format-specific processing stops there.

The image indexer sends recognized images to Poly's image loader. That loader has dedicated paths for HEIF, PSD, JPEG XL, and SVG, then sends other image types to a general decoder. OpenRaster has no dedicated unpacking path, and the general decoder cannot treat its ZIP container as a bitmap. As a result:

• Poly stores and manages the file, and filename search still works.
• It does not extract mergedimage.png, thumbnail.png, stack.xml, layer names, dimensions, or embedded metadata.
• It does not generate WebP thumbnails or visual-similarity embeddings.
• The image viewer and Poly agent do not receive a rendered composite.

This is a decoder limitation, not a limitation of OpenRaster itself. The file already contains both a thumbnail and a merged PNG, but Poly does not currently extract either one.

Converting OpenRaster without losing the wrong thing

Choose the output based on whether editable layers matter:

• Another layered editor format: Open the file in Krita or GIMP, review the result, then save in the target editor's native project format. Unsupported blend modes or extensions may render differently, so compare the composite before discarding the .ora source.
• PNG: Export the merged image for lossless pixels and transparency. PNG does not preserve OpenRaster's editable layer stack.
• JPEG: Use it for a smaller photograph-like final image when transparency and layers are no longer needed. JPEG encoding is lossy.
• WebP or AVIF: Use these for web delivery after flattening. They are delivery formats, not substitutes for an editable OpenRaster project.

Extracting mergedimage.png is the safest quick conversion when the supplied composite is trustworthy. Rebuilding the image by combining individual layer PNGs requires honoring the order, offsets, opacity, visibility, groups, isolation, and compositing operations in stack.xml.³

OpenRaster compared with PSD and native project files

OpenRaster's simplicity makes it approachable, but interchange always involves a lowest-common-denominator tradeoff. The baseline specification defines a focused set of compositing modes and permits extensions. An application that does not understand an extension may preserve the flattened appearance while losing editability or feature semantics.³⁸

Security and privacy considerations

Treat an .ora file as an untrusted archive. A safe reader should limit expanded size and entry count, reject paths that escape the extraction directory, validate stack.xml, and impose sensible image-dimension and XML-complexity limits. The outer ZIP header alone does not establish that the required entries are present or valid.

OpenRaster can also contain material that is invisible in the flattened result, including hidden layers, layer names, discarded sketches, and application-specific extension data. Review the complete layer stack before sharing an editable source file. Exporting a clean flattened PNG is often better when recipients only need the finished artwork.