A new app will be available soon that enables you to play media files from either an attached USB device or from a network connected home media server that supports DLNA protocols. The preview version of the Media Player app will initially only support USB devices, with DLNA support coming soon. Xbox One will support more formats than Xbox 360, including support for dozens of new file formats like mpeg 2 TS, animated gifs and mkv which will be added by the end of the year. See below for additional file formats.

Xbox One compatible file types:

3gp audio
3gp video
3gp2
aac
adts
animated gif
asf
avi divx
avi dv
avi uncompressed
avi xvid
bmp
jpg
gif
h264 avchd
mjpeg
mkv
mov
mp3
mpeg 1 ps
mpeg 2
mpeg 2 hd
mpeg 2 ts
mpeg 4 h264 aac
mpeg 4 sp
png
tiff
wav
wma
wma lossless
wma pro
wma voice
wmv
wmv hd

Xbox One Media Player and Compatible File Types Revealed
- http://www.xboxone-hq.com/news/224-xbox-one-media-player-and-compatible-file-types-revealed

sounds just like the xbox 360 launch, i remember having to wait ages before i could play divx files or whatever.