Using Microsoft Office MIME Types with MailKit

Recently I had problems with attaching a macro-enabled Excel workbook to an email. When it was received, the macro’s didn’t run correctly on the latest version of Microsoft Office. It did run correctly on Excel 2007. The workbook was being corrupted in the encoding into the email or decoding when its downloaded.

The following code will maintain the integrity of the Excel workbook when attached to an e-mail:

ContentType xlsmContent = new ContentType ("application/vnd.ms-excel.sheet.macroEnabled.12", "octet-steam");
builder.Attachments.Add ($"{Directory.GetCurrentDirectory ()}/Temp/workbook.xlsm", xlsmContent);

These are the MIME types for documents in Microsoft Office: