As detailed here, you can see a few different MIME types that could be used. The following is that list:
application/vnd.ms-project, application/msproj, application/msproject, application/x-msproject, application/x-ms-project, application/x-dos_ms_project, application/mpp, zz-application/zz-winassoc-mpp
NOTE: "MIME type for [File Extension]" is a good phrase to use when searching for these things