I can confirm that this happens.
Yesterday my EX2 started to build thumbnails for jpeg photos stored on EX2 I have disabled almost all features of EX2 (I do not use things like Cloud access, iTunes, DLNA etc) but Convert still build thumbnails for photos.
I also would like to disable it because I will never benefit from thumbnails, since I use EX2 only as a storage device.
I saw that someone submitted Idea to improve My Cloud:
http://community.wd.com/t5/Personal-Cloud-Ideas/Turn-off-Convert-Process/idi-p/712550
Maybe It is worth voting J
