How can i cast unsupported files from "WD MyCloud 2tb" to chromecast?

Zakari wrote:

… but there is a twonky media server installed on the device, why can’t it do the job? 

Because the Twonky media server is not designed to transcode media files. The other reason that the entry level WD My Cloud devices are poor devices for transcoding is their low powered ARM processor and low amount RAM.

Plex does have a module that can be add to the (see the  Plex NAS compatibility list for more information on supported NAS devices) but the lower cost WD My Cloud devices are not officially supported. Currently Plex only supports the following WD My Cloud models and even then not all models support transcoding:

Western Digital My Cloud EX4
Western Digital My Cloud Mirror
Western Digital My Cloud EX2
Western Digital My Cloud EX2100
Western Digital My Cloud EX4100
Western Digital My Cloud DL2100
Western Digital My Cloud DL4100

if you need to transcode media files then its best to setup a dedicated PC that has the capability to adequately support transcoding on the fly. Otherwise use free conversion programs like  Handbrake and others to batch convert your media files into a format that both Twonky and your DLNA clients can support prior to watching the content.

Edit to add: Another program that may possibly work to transcode on the fly to Chomecast from the WD My Cloud is the  BubbleUPnP Server but it too also requires the use of a PC/Mac/Linux or supported NAS (Synology and QNAP).

There is some discussion from earlier today on using BubbleUPnP Server with the WD My Cloud in the following thread.

Remote media access with BubbleUPnP