WebKit's IndexedDB cannot store Blobs in private contexts and aborts the write with 'Error preparing Blob/File data to be stored in object store'. Attachment previews and tool change images are Blobs, so after the first image or document-changing reply every save of the conversation failed. Messages are now stored with each Blob as its type and bytes, converted before the transaction opens and back on read; Blobs saved earlier still read as they are. A WebKit test, which runs in a private context, fails without this and passes with it.
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