Expose LocalDocs file extensions

[MrLoompa]

I see that the file extensions for the LocalDocs are hardcoded in here:

gpt4all/gpt4all-chat/database.cpp

Line 893 in baf1dfc

static const QList<QString> extensions { "txt", "pdf", "md", "rst" };

Maybe would be nice to expose this in the plugin UI.
Simplest use: Reading local code.

[cebtenzzre]

Duplicate of #855