Custom Attachment Content Type

I'm working on a OWA addin and I need to create an attachment with a custom mime type that will be processed by a server later. Right now, the attachment APIs don't allow me to provide a custom mime type for the file, always relying on the file extension.

Given that the the mapping for extensions are not even actively maintained (as noted https://docs.microsoft.com/en-us/javascript/api/outlook/office.attachmentdetails?view=outlook-js-preview#contentType), ideally we could have an option to set the file mimetype when creating the attachment. The easiest way I can see this happening is adding an option to the attachment creation methods that allow a custom mimetype to be defined (example below);

base64String = "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";
Office.context.mailbox.item.addFileAttachmentFromBase64Async(
    base64String,
    "logo.png",
    { isInline: false, contentType: "image/png" },
    function(result) { console.log(result); });

Thanks