Are you sure it's capital T and not lowercase t?

>From man:
      -t fs-type
             Specify the filesystem type (i.e., ext2, ext3, ext4, etc.) that is to be created.  If this  option  is  not
             specified,  mke2fs will pick a default either via how the command was run (for example, using a name of the
             form mkfs.ext2, mkfs.ext3, etc.) or via a default as defined by the /etc/mke2fs.conf(5) file.   This option
             controls  which  filesystem  options  are  used  by  default,  based on the fstypes configuration stanza in
             /etc/mke2fs.conf(5).

             If the -O option is used to explicitly add or remove filesystem options that should be  set  in  the  newly
             created filesystem, the resulting filesystem may not be supported by the requested fs-type.  (e.g., "mke2fs
             -t ext3 -O extents /dev/sdXX" will create a filesystem that is not supported by the ext3 implementation  as
             found  in  the  Linux  kernel; and "mke2fs -t ext3 -O ^has_journal /dev/hdXX" will create a filesystem that
             does not have a journal and hence will not be supported by the ext3 filesystem code in the Linux kernel.)

      -T usage-type[,...]
             Specify how the filesystem is going to be used, so that mke2fs can choose optimal filesystem parameters for
             that  use.   The  usage types that are supported are defined in the configuration file /etc/mke2fs.conf(5).
             The user may specify one or more usage types using a comma separated list.

             If this option is is not specified, mke2fs will pick a single default usage type based on the size  of  the
             filesystem to be created.  If the filesystem size is less than or equal to 3 megabytes, mke2fs will use the
             filesystem type floppy.  If the filesystem size is greater than 3 but less than or equal to 512  megabytes,
             mke2fs(8)  will  use  the  filesystem  small.   Otherwise,  mke2fs(8)  will use the default filesystem type
             default.