This is the error message from update-desktop-database when MimeType of file-roller.desktop contained a space:
File '/usr/share/applications/file-roller.desktop' contains invalid MIME
type ' application/x-gtar' that contains invalid characters

The desktop entry validated just fine with desktop-file-utils 0.15 on Mandriva Cooker. 

$ fgrep MimeType file-roller.desktop
MimeType=application/x-7z-compressed;application/x-7z-compressed-tar;application/x-ace;application/x-alz;application/x-ar;application/x-arj;application/x-bzip;application/x-bzip-compressed-tar;application/x-bzip1;application/x-bzip1-compressed-tar;application/x-cabinet;application/x-cbr;application/x-cbz;application/x-cd-image;application/x-compress;application/x-compressed-tar;application/x-cpio;application/x-deb;application/x-ear;application/x-executable; application/x-gtar;application/x-gzip;application/x-gzpostscript;application/x-java-archive;application/x-lha;application/x-lhz;application/x-lzma;application/x-lzma-compressed-tar;application/x-lzop;application/x-lzop-compressed-tar;application/x-rar;application/x-rar-compressed;application/x-rpm;application/x-rzip;application/x-tar;application/x-tarz;application/x-stuffit;application/x-war;application/x-zip;application/x-zip-compressed;application/x-zoo;application/zip;multipart/x-zip;
$ desktop-file-validate file-roller.desktop
$