1) Download the latest rpmforge-release rpm from

http://apt.sw.be/redhat/el6/en/x86_64/rpmforge/RPMS/

2) Install rpmforge-release rpm:

rpm -Uvh rpmforge-release*rpm

3) install MediaTomb via yum:

yum install mediatomb

4) open /etc/mediatomb/config.xml in your text editor of choice
5) remove the comment tags around the <custom-http-headers> section and make that section look like this:

<custom-http-headers>
   <add header="transferMode.dlna.org: Streaming"/>
   <add header="contentFeatures.dlna.org: DLNA.ORG_OP=01;DLNA.ORG_CI=0;DLNA.ORG_FLAGS=017000 00000000000000000000000000"/>
</custom-http-headers>

6) Under <mappings>, add

<map from="avi" to="video/mpeg"/>
<map from="mkv" to="video/mpeg"/>

7) save and close config.xml and restart mediatomb

sudo /etc/init.d/mediatomb restart

8) load MediaTomb web interface and tell it where to start looking for files.