The Source Code Viewer plugin and the Text viewer plugin have the setting Catch-all . which cannot be changed (see the attached screen-shots). This makes makes for example .doc files to be viewed in Source Code Viewer and .flac files to be viewed in the Text viewer!
However the Source Code Viewer does work well with source code files (for example .pas).
So how can I customize the file-types to be used with Source Code Viewer and Text viewer plugin?
You can't, but since Catch-Alls appear at the bottom of the plugins list they will only be used if no other plugin, and no internal viewer, can handle the files. For example, there is no viewer for FLAC files. (The FLAC plugin extracts tags and data from FLAC files to populate the audio-related columns in the file display, but it doesn't provide any kind of viewer in the viewer window, at least not right now.)
Normally, if a file cannot be displayed by anything else it will drop through to the Text plugin which will display it as text (if it looks like a text file) or as a hex view (if it looks like binary data).
You may be able to change what the Source Code Viewer plugin displays by editing its XML config file. I think there's some info about it in the thread for the plugin. It's been a while since I have used it so I'm not completely sure. (I stopped using it since it tries to handle all text files but when given a Unicode text file shows an error message, instead of telling Opus that it can't view it so that it drops to the Text plugin which can.)
If .doc files are ending up in either of those two viewers then the ActiveX and MultiView plugins must both be failing to display them for some reason. (They both depend on additional components on your system, either Office, ActiveX controls or QuickView DLLs.)