How can I store custom parts with mastra mongodb storage

[daneatmastra]

This issue was created from Discord post 1446411042447032411:

Hello, i want my user can attach file-reference ( not attachment directly )

                        ...message.parts,
                        ...(attachments
                            ? attachments.map((attachment) => ({
                                    type: 'file-reference',
                                    fileId: attachment.fileId,
                                    fileName: attachment.fileName
                                }))
                            : [])
                    ]```

But the parts "file-reference" is not stored in mongodb .How can i achieve this ?