Please first update the firmware to 3.5.1 or later which TwonkyMedia Server have been updated to v6.

For .avi/.divx files, TwonkyMedia server checks if a file with the same name and one of
the following extensions (.srt/.sub/.txt/.smi/.ssa/.psb/.ass) exists. 

Constraints: 
File name comparison is case insensitive. Therefore the files "file1.avi" and "FILE1.SRT"
are considered matching.

If multiple subtitle/caption files are available for a video item, the server will include only
one of the subtitle/caption files in the <res> element. Which one this will be, depends on
the order of entries in the file system directory structure, so it is undefined – as a user in
general only want to see one language in subtitels this is not a real issue.

Applied Models: Arm-based NAS series, Intel x86-based NAS series