What causes "image MIME type is missing" in siyuan-note/siyuan?

Set MIMEType (e.g. "image/png") on every attachment that carries Data

image MIME type is missing

Error message

image MIME type is missing

What it means

resolveAgentAttachment requires a MIME type whenever raw attachment Data is present, because the data cannot be turned into a valid image content part for the model without knowing its type. A missing MIMEType with non-empty Data produces this error.

Source

Thrown at kernel/agent/attachments.go:146

	if imageCount == 0 {
		return openai.ChatCompletionMessage{}, false
	}
	if omitted {
		parts = append(parts, openai.ChatMessagePart{
			Type: openai.ChatMessagePartTypeText,
			Text: "One or more image attachments were omitted because they were unavailable or exceeded the request limit.",
		})
	}
	return openai.ChatCompletionMessage{
		Role:         openai.ChatMessageRoleUser,
		MultiContent: parts,
	}, true
}

func resolveAgentAttachment(attachment AgentAttachment) (data []byte, mimeType string, width, height int, err error) {
	if len(attachment.Data) > 0 {
		if attachment.MIMEType == "" {
			err = fmt.Errorf("image MIME type is missing")
			return
		}
		data = attachment.Data
		mimeType = attachment.MIMEType
		width = attachment.Width
		height = attachment.Height
		return
	}
	var prepared kernelModel.PreparedDocumentImage
	var prepareErr error
	if attachment.DocumentID == "" {
		prepared, prepareErr = kernelModel.PrepareAgentMessageImage(attachment.Path)
	} else {
		prepared, prepareErr = kernelModel.PrepareDocumentImage(attachment.DocumentID, attachment.Path)
	}
	if prepareErr != nil {
		err = prepareErr
		return

View on GitHub (pinned to 8641553a1f)

Solutions

Set MIMEType (e.g. "image/png") on every attachment that carries Data
Detect the MIME type from the file signature or the source paste event before sending
Default missing MIME types to "image/png" at the attachment-production site if that matches the data
Validate attachments in the frontend before posting them to the agent

Example fix

// before
attach := AgentAttachment{Data: pngBytes}
// after
attach := AgentAttachment{Data: pngBytes, MIMEType: "image/png"}

Defensive patterns

Strategy: validation

Validate before calling

function isValidAttachment(a) {
  return !(a.data && a.data.length > 0) || Boolean(a.mimeType)
}

Type guard

function hasMIME(a) { return typeof a.MIMEType === "string" && a.MIMEType !== "" }

Prevention

Always populate MIMEType when constructing AgentAttachment values
Detect MIME from magic bytes at paste/import time
Add a constructor/helper that rejects attachments without a MIME type

When it happens

Trigger: buildAttachmentMessage calls resolveAgentAttachment with an AgentAttachment whose Data is non-empty but MIMEType is "" — the named-return err is set and the attachment resolution aborts.

Common situations: Frontend or plugin builds AgentAttachment structs manually and forgets MIMEType; attachments deserialized from older/external payloads that omit the MIME field; clipboard pastes where the MIME type was not detected.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of siyuan-note/siyuan@8641553a1f (2026-09-11). Data as JSON: /api/errors/8d79b460ae0d1961. Report an issue: GitHub.