My /etc/mime.types says this:
application/vnd.oasis.opendocument.text for *.odtapplication/vnd.oasis.opendocument.spreadsheet for *.odsapplication/vnd.oasis.opendocument.presentation for *.odp
This makes sense because the corporate standard (vnd) developed by OASIS is used for different opendocuments formats.
If you don't want to worry about sending the correct mime types, you can use the finfo class to do this for you:
$finfo = new finfo(FILEINFO_MIME); header('Content-Type: ' . $finfo->file('/path/to/file'));
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