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 here), 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 = "iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAACXBIWXMAAAsSAAALEgHS3X78AAACRUlEQVRYw82XzXHbMBCFP2F8tzsQc8Ixyh0zoiuIXIGdCsxUYKqC0B04FdiuwMoM7mGOOIXqQGoAymXhgSX+itJM9kIRFLAP+3YXD5Pdbscx5oxaAIW8Ztr6l2PWmQwF4IyaieP53qdfAqQ8CwBn1JU4vpWhrbxXQA5MZfynANmcDIAzKgcy4FKGXsVJFf3nLgKyBQptfT4KQMRz2N0fcbxqmRMDWXflx0VPnrdArq0vekQ1Dv0UeHZGNebHhwjU8AzwKM43RyZnbAf58Q6ghudeWd0Aus0+5EcMIIRi3beua0D3Nm39BEAx3i7HTK4DEBJn5YxKOnaRA5+ErpMBWMpzDvx1RuXCcxOISlufAjfC7zgAsqsvUvMAD0ApPaEtGi9AIlUzKgJo60tt/SyKRkzLrAXERluf7W1gOICWaMyB386oooOWsIHvXbSoHuUSFovtHqicUVnH3EJoeT0aQEf5/XBGlc6otIOWBXAtPeZkAIJ9Bt6cUU9tZautX2nrk3MACHYr1ZKProKRtDw4o8pzAPjWo+NtpXTTvoteDDg8noDAcwbcRedAkGdFXyk2GEDcegVAFp2gyVDHjRQ4o6q2smoqtR5Hd+qMqtoALCWUUymr1m43QMZfOaMK4C0SrMsDANJ2E5FNcbdbjHC+ENl+H0myJFbLtaq4Rt8dyPBYRQV1E40nMv9rl7xrOw3DGb+Whcqu3i/OM6CUOWvgRlufNmnLYy4m77uJI7AXtdNcTDrU71LEyv7v01/N/ovL6bmu5/8A1tNWZldH0W4AAAAASUVORK5CYII=";
Office.context.mailbox.item.addFileAttachmentFromBase64Async(
    base64String,
    "logo.png",
    { isInline: false, contentType: "image/png" },
    function(result) { console.log(result); });

Thanks