Automate/ Contact sheet II. Is there a way to not have the file extension in the captions?

An alternative to editing the ContactSheetII.jsx file is to run a script to remove filename extensions from all text layers:

#target photoshop
// Remove filename extension from all text layers

for (var i = 0; i < app.documents.length; i++) {
    app.activeDocument = app.documents[i];
    var doc = app.activeDocument;
    for (var j = 0; j < doc.layers.length; j++) {
        try {
            doc.activeLayer = doc.layers[j];
            if (app.activeDocument.activeLayer.kind == LayerKind.TEXT) {
                doc.activeLayer.textItem.contents = doc.activeLayer.textItem.contents.replace(/\.[^\.]+$/, '');
            }
        } catch (e) {}
    }
}

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html

If you use Bridge > Output tab or workspace, you can omit the file extension by unchecking the option Include File Extension in the Document section of the Output panel.