What causes "unsupported image media type: ${mediaType || '(empty)'}" in deepseek-ai/deepseek-harness?

Validate file.type against the four accepted MIME values before calling createDraftImages and reject unsupported files with a user notice

deepseek-ai/deepseek-harness · error · UnsupportedImageMediaTypeError

unsupported image media type: ${mediaType || '(empty)'}

Error message

unsupported image media type: ${mediaType || '(empty)'}

What it means

imageMediaType() is the single MIME whitelist for composer images: image/png, image/jpeg, image/webp, image/gif. Any other value — including the empty string, displayed as '(empty)' — throws UnsupportedImageMediaTypeError. It runs inside createDraftImages() before any attachment is registered, and again inside encodeImage() during serialization.

Source

Thrown at packages/client/ui-conversation/src/client/service.ts:358

  /** Canonical base64 wire form of one browser image file. */
  private async encodeImage(file: File): Promise<SubmitImageAttachment> {
    return {
      mediaType: imageMediaType(file.type),
      data: bytesToBase64(new Uint8Array(await file.arrayBuffer())),
      ...(file.name === '' ? {} : { name: file.name }),
    }
  }
}

function imageMediaType(value: string): ImageMediaType {
  switch (value) {
    case 'image/png':
    case 'image/jpeg':
    case 'image/webp':
    case 'image/gif':
      return value
    default:
      throw new UnsupportedImageMediaTypeError(value)
  }
}

function bytesToBase64(data: Uint8Array): string {
  let binary = ''
  const chunk = 0x8000
  for (let offset = 0; offset < data.length; offset += chunk) {
    binary += String.fromCharCode(...data.subarray(offset, offset + chunk))
  }
  return btoa(binary)
}

function revokePreview(url: string): void {
  if (url.startsWith('blob:')) URL.revokeObjectURL(url)
}

View on GitHub (pinned to b150a551b8)

Solutions

Validate file.type against the four accepted MIME values before calling createDraftImages and reject unsupported files with a user notice
Transcode HEIC/AVIF to PNG or JPEG client-side (canvas or codec library) before attaching
Give synthetic Files an explicit accepted MIME type

Example fix

// before
const attachments = service.createDraftImages(files)
// after
const supported = files.filter(f => SUPPORTED_IMAGE_TYPES.has(f.type))
notifySkipped(files.length - supported.length)
const attachments = service.createDraftImages(supported)

Defensive patterns

Strategy: type-guard

Validate before calling

const rejected = files.filter(f => !isSupportedImageType(f))
if (rejected.length > 0) notifyUnsupported(rejected)

Type guard

const SUPPORTED_IMAGE_TYPES = new Set(['image/png', 'image/jpeg', 'image/webp', 'image/gif'])
function isSupportedImageType(file: File): file is File & { type: 'image/png' | 'image/jpeg' | 'image/webp' | 'image/gif' } {
  return SUPPORTED_IMAGE_TYPES.has(file.type)
}

Try / catch

try {
  return service.createDraftImages(files)
} catch (error) {
  if (error instanceof UnsupportedImageMediaTypeError) return rejectWithNotice(files, error)
  throw error
}

Prevention

Filter at the file picker and drop zone, not at submit time
Transcode HEIC and AVIF sources to PNG or JPEG before attaching
Never construct File objects without an explicit accepted MIME type

When it happens

Trigger: Passing a File whose type is image/heic (iPhone photos), image/avif, image/svg+xml, application/octet-stream, or '' (extensionless blobs, synthetic Files built without a MIME type) into createDraftImages(); or feeding such a file to encodeImage().

Common situations: iOS Safari uploads defaulting to HEIC; AVIF exports from modern image tools; dragged-in SVGs; test fixtures constructed as new File([bytes], 'x') with no type option.

Related errors

AI-assisted analysis of deepseek-ai/deepseek-harness@b150a551b8 (2026-08-24). Data as JSON: /api/errors/1436c6c3bda95884. Report an issue: GitHub.