The following aggregation
{
$match : {
filename :

{ $in : defaultFilesArray }

}
}, {
$project :

{ filename : 1, _id : 0 }

}, {
$group : {
_id : 1,
pages :

{ $push : '$filename' }

}
}

Should output an array with a single element, containing the array 'pages' and this array would contain the strings for the filenames.
However, when toArray is used on the resulting cursor, I get the following:

[

{ "filename": "/404.html" }

,

{ "filename": "/audioGenericThumb.png" }

,

{ "filename": "/defaultBanner.png" }

,

{ "filename": "/genericThumb.png" }

,

{ "filename": "/login.html" }

,

{ "filename": "/maintenance.html" }

,

{ "filename": "/maintenanceImage.png" }

,

{ "filename": "/spoiler.png" }

]

It seems the group stage is not being processed at all.