diff --git a/apps/desktop/src/image-drag-files.ts b/apps/desktop/src/image-drag-files.ts
index 0b01ea497c..be690aa59b 100644
--- a/apps/desktop/src/image-drag-files.ts
+++ b/apps/desktop/src/image-drag-files.ts
@@ -3,12 +3,14 @@ import { randomUUID } from 'node:crypto'
 import { mkdtemp, rm, writeFile } from 'node:fs/promises'
 import { join } from 'node:path'
 
-const IMAGE_EXTENSIONS: Readonly<Record<string, string>> = {
+// Keep supported MIME types in sync with IMAGE_MEDIA_TYPES in
+// packages/client/ui-sidebar-documentpreview/src/client/image/ImageBody.tsx (extensions: adjacent index.ts).
+const DRAG_FILE_EXTENSION_BY_MIME: Readonly<Record<string, string>> = {
   'image/png': 'png', 'image/jpeg': 'jpg', 'image/gif': 'gif', 'image/webp': 'webp',
   'image/bmp': 'bmp', 'image/x-icon': 'ico', 'image/vnd.microsoft.icon': 'ico', 'image/svg+xml': 'svg',
 }
 // Bounds aggregate original bytes in preparation and retained temporary files.
-const MAX_IMAGE_BYTES = 64 * 1024 * 1024
+const MAX_TOTAL_DRAG_FILE_BYTES = 64 * 1024 * 1024
 
 interface ImageDragFile {
   readonly directory: string
@@ -36,16 +38,17 @@ export class ImageDragFiles {
    * @returns application-local opaque handle; failures retain no usable handle or partial file.
    */
   prepare(bytes: unknown, name: unknown, mediaType: unknown): Promise<string> {
-    const extension = typeof mediaType === 'string' && Object.hasOwn(IMAGE_EXTENSIONS, mediaType)
-      ? IMAGE_EXTENSIONS[mediaType] : undefined
-    if (this.disposed || !(bytes instanceof Uint8Array) || bytes.byteLength === 0 || bytes.byteLength > MAX_IMAGE_BYTES - this.stagedBytes
-      || typeof name !== 'string' || typeof mediaType !== 'string' || extension === undefined) {
+    const outputExtension = typeof mediaType === 'string' && Object.hasOwn(DRAG_FILE_EXTENSION_BY_MIME, mediaType)
+      ? DRAG_FILE_EXTENSION_BY_MIME[mediaType] : undefined
+    if (this.disposed || !(bytes instanceof Uint8Array) || bytes.byteLength === 0
+      || bytes.byteLength > MAX_TOTAL_DRAG_FILE_BYTES - this.stagedBytes
+      || typeof name !== 'string' || typeof mediaType !== 'string' || outputExtension === undefined) {
       return Promise.reject(new Error('Invalid image transfer'))
     }
     let stem = name.replace(/\.[^.]*$/u, '').replace(/[<>:"/\\|?*\x00-\x1f]/gu, '_').replace(/^[. ]+|[. ]+$/gu, '').slice(0, 120) || 'image'
     if (/^(con|prn|aux|nul|com[1-9¹²³]|lpt[1-9¹²³])(?:\.|$)/iu.test(stem)) stem = `_${stem}`
     this.stagedBytes += bytes.byteLength
-    const operation = this.write(bytes, `${stem}.${extension}`).catch((error: unknown) => {
+    const operation = this.write(bytes, `${stem}.${outputExtension}`).catch((error: unknown) => {
       this.stagedBytes -= bytes.byteLength
       throw error
     })
diff --git a/packages/client/ui-attachment/src/client/drop-events.ts b/packages/client/ui-attachment/src/client/drop-events.ts
index 3d08df15ff..67dc743bf5 100644
--- a/packages/client/ui-attachment/src/client/drop-events.ts
+++ b/packages/client/ui-attachment/src/client/drop-events.ts
@@ -1,5 +1,5 @@
 /** Document drag-and-drop listeners owned by one mounted attachment view. */
-import { IMAGE_DRAG_TYPE } from '@deepseek-ai/dsh-client-ui-primitives'
+import { IMAGE_DRAG_TOKEN_TYPE } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { ComposerAttachmentsProps } from '@deepseek-ai/dsh-client-ui-conversation/client'
 
 /**
@@ -38,7 +38,7 @@ export function installDocumentDropEvents(
 ): () => void {
   const fileTransfer = (event: globalThis.DragEvent): DataTransfer | null => {
     const dataTransfer = event.dataTransfer
-    if (dataTransfer === null || (!dataTransfer.types.includes('Files') && !dataTransfer.types.includes(IMAGE_DRAG_TYPE))) return null
+    if (dataTransfer === null || (!dataTransfer.types.includes('Files') && !dataTransfer.types.includes(IMAGE_DRAG_TOKEN_TYPE))) return null
     return dataTransfer
   }
   const reset = (): void => {
@@ -50,7 +50,7 @@ export function installDocumentDropEvents(
     if (dataTransfer === null) return
     event.preventDefault()
     dragDepth.current += 1
-    if (!dataTransfer.types.includes(IMAGE_DRAG_TYPE)) setDragActive(true)
+    if (!dataTransfer.types.includes(IMAGE_DRAG_TOKEN_TYPE)) setDragActive(true)
   }
   const onDragOver = (event: globalThis.DragEvent): void => {
     const dataTransfer = fileTransfer(event)
@@ -72,7 +72,7 @@ export function installDocumentDropEvents(
     event.preventDefault()
     reset()
     if (canAcceptDrop) {
-      const image = dataTransfer.types.includes(IMAGE_DRAG_TYPE) ? readImageDrag(dataTransfer) : undefined
+      const image = dataTransfer.types.includes(IMAGE_DRAG_TOKEN_TYPE) ? readImageDrag(dataTransfer) : undefined
       const files = image === undefined ? [...dataTransfer.files] : [image]
       if (files.length > 0) onAddFiles(files, droppedDirectories(dataTransfer, files))
     }
diff --git a/packages/client/ui-primitives/src/ImageDragProvider.tsx b/packages/client/ui-primitives/src/ImageDragProvider.tsx
index 2027011017..0d335f78dc 100644
--- a/packages/client/ui-primitives/src/ImageDragProvider.tsx
+++ b/packages/client/ui-primitives/src/ImageDragProvider.tsx
@@ -3,8 +3,8 @@ import { createContext, useCallback, useContext, useEffect, useMemo, useRef, use
 import { Toast } from './Toast.tsx'
 import type { DesktopImageDrag } from './image-drag.ts'
 
-/** Browser drag token; file bytes remain in the owning mounted application. */
-export const IMAGE_DRAG_TYPE = 'application/x-dsh-image'
+/** DataTransfer type for an application-local drag token; file bytes remain in the owning mounted application. */
+export const IMAGE_DRAG_TOKEN_TYPE = 'application/x-dsh-image'
 
 interface ImageTransfer {
   desktop: DesktopImageDrag | undefined
@@ -43,7 +43,7 @@ export function ImageDragProvider({ children }: { readonly children: ReactNode }
   }, [end])
   const read = useCallback((transfer: DataTransfer): File | undefined => {
     const current = active.current
-    return current !== undefined && current.token === transfer.getData(IMAGE_DRAG_TYPE) ? current.file : undefined
+    return current !== undefined && current.token === transfer.getData(IMAGE_DRAG_TOKEN_TYPE) ? current.file : undefined
   }, [])
   const value = useMemo(() => ({ desktop, report, begin, end, read }), [desktop, report, begin, end, read])
   return <ImageDragFeedback.Provider value={value}>
diff --git a/packages/client/ui-primitives/src/image-drag.ts b/packages/client/ui-primitives/src/image-drag.ts
index ad56f51b25..94714ae2ce 100644
--- a/packages/client/ui-primitives/src/image-drag.ts
+++ b/packages/client/ui-primitives/src/image-drag.ts
@@ -1,7 +1,7 @@
 /** Original image files supplied to browser and native drag sessions. */
 import { useCallback, useEffect, useRef, type DragEvent, type MouseEvent, type PointerEvent } from 'react'
 
-import { IMAGE_DRAG_TYPE, useImageDragTransfer } from './ImageDragProvider.tsx'
+import { IMAGE_DRAG_TOKEN_TYPE, useImageDragTransfer } from './ImageDragProvider.tsx'
 
 /** Application-owned native image transfer; prepared handles never expose host paths. */
 export interface DesktopImageDrag {
@@ -106,7 +106,7 @@ export function useImageDrag(src: string, name: string, failureText: string, ori
       void desktop.start(prepared.handle).then((started) => { if (!started) failed() }, failed)
       return
     }
-    event.dataTransfer.setData(IMAGE_DRAG_TYPE, transfer.begin(prepared.file))
+    event.dataTransfer.setData(IMAGE_DRAG_TOKEN_TYPE, transfer.begin(prepared.file))
     event.dataTransfer.setData('DownloadURL', `${prepared.file.type}:${prepared.file.name}:${src}`)
   }, [src, desktop, prepare, reportFailure, failureText, transfer])
   const onPointerDown = useCallback((event: PointerEvent<HTMLImageElement>): void => {
diff --git a/packages/client/ui-primitives/src/index.ts b/packages/client/ui-primitives/src/index.ts
index bdd9d37e14..37e04c877f 100644
--- a/packages/client/ui-primitives/src/index.ts
+++ b/packages/client/ui-primitives/src/index.ts
@@ -67,7 +67,7 @@ export { pointerModality } from './input-modality.ts'
 export { Toast } from './Toast.tsx'
 export { fileSizeText } from './file-size.ts'
 export { writeClipboard } from './clipboard.ts'
-export { IMAGE_DRAG_TYPE, ImageDragProvider, ImageDrop } from './ImageDragProvider.tsx'
+export { IMAGE_DRAG_TOKEN_TYPE, ImageDragProvider, ImageDrop } from './ImageDragProvider.tsx'
 export { DraggableImage } from './DraggableImage.tsx'
 export type { DraggableImageProps } from './DraggableImage.tsx'
 export type { DesktopImageDrag } from './image-drag.ts'
diff --git a/packages/client/ui-primitives/tests/image-drag.client.spec.tsx b/packages/client/ui-primitives/tests/image-drag.client.spec.tsx
index 712683b863..d2561d77b5 100644
--- a/packages/client/ui-primitives/tests/image-drag.client.spec.tsx
+++ b/packages/client/ui-primitives/tests/image-drag.client.spec.tsx
@@ -3,7 +3,7 @@ import { useEffect } from 'react'
 import { File as NodeFile } from 'node:buffer'
 import { act, cleanup, fireEvent, render, screen } from '@testing-library/react'
 import { afterEach, expect, it, vi } from 'vitest'
-import { IMAGE_DRAG_TYPE, ImageDragProvider, useImageDrop } from '../src/ImageDragProvider.tsx'
+import { IMAGE_DRAG_TOKEN_TYPE, ImageDragProvider, useImageDrop } from '../src/ImageDragProvider.tsx'
 import { DraggableImage } from '../src/DraggableImage.tsx'
 import { useImageDrag, type DesktopImageDrag } from '../src/image-drag.ts'
 
@@ -36,7 +36,7 @@ it('hands original bytes to the browser without alt text or an internal URI payl
   await act(async () => { fireEvent.dragStart(screen.getByRole('img'), { dataTransfer }) })
   expect(dataTransfer.items.add).not.toHaveBeenCalled()
   expect(dataTransfer.items.clear).toHaveBeenCalledOnce()
-  expect(dataTransfer.setData).toHaveBeenCalledWith(IMAGE_DRAG_TYPE, expect.any(String))
+  expect(dataTransfer.setData).toHaveBeenCalledWith(IMAGE_DRAG_TOKEN_TYPE, expect.any(String))
   expect(dataTransfer.effectAllowed).toBe('copy')
   expect(dataTransfer.setData).toHaveBeenCalledWith('DownloadURL', 'image/png:original.png:blob:original')
   fireEvent.click(screen.getByRole('img'), { detail: 1 })
@@ -169,7 +169,7 @@ it('fetches a readable source once and rejects a foreign drag token', async () =
   expect(fetchImage).toHaveBeenCalledOnce()
   const dataTransfer = transfer()
   fireEvent.dragStart(screen.getByRole('img'), { dataTransfer })
-  dataTransfer.setData(IMAGE_DRAG_TYPE, 'foreign')
+  dataTransfer.setData(IMAGE_DRAG_TOKEN_TYPE, 'foreign')
   fireEvent.drop(screen.getByTestId('drop'), { dataTransfer })
   expect(receive).toHaveBeenCalledWith(undefined)
 })