First if you haven’t done so already, read the unofficial Twonky FAQ:
https://community.wd.com/t/faq-twonky-dlna-media-server-setup-use/95373
Next, The Twonky DLNA server only supports the following file formats.
Audio Files:
3GP
AAC
AC3
AIF
FLAC
LPCM
M4A
M4B
MP1
MP2
MP3
MP4
MPA
OGG
WAV
WMA
Video Files:
3GP
ASF
AVI
DivX
DV
DVR-MS
FLV
M1V
M4V
MKV
MOV
MP1
MP4
MPE
MTS
MPV
MPG
MPEG1
MPEG2
MPEG4
M2TS
M4P
M4V
QT
QTI
QTIF
RM
SPTS
VDR
VOB
WMV
Xvid
Image Files:
BMP
JPEG
PNG
TIF
Certain files, like MP4 are container files and can be encoded in such a way that Twonky will not properly read them and will classify them as audio. The workaround is to reencode the media file into a Twonky supported format using programs like the free Handbrake (https://handbrake.fr/).
In other cases incorrect metadata for the media (MP3/MP4 for example) can cause problems. One can use metadata editors like the free MediaMonkey (http://www.mediamonkey.com/) or the free MP3Tag (http://www.mp3tag.de/) to edit/modify/remove metadata tags on certain media files.
