The "contentstore" is where all course assets are stored. It's really a wrapper of code around a GridFS (MongoDB) backend and it stores binary files which can PDFs, WAVs, JPGs, or other. The contentstore code is mainly here:

edx-platform/common/lib/xmodule/xmodule/contentstore

Counting Assets in Each Course

If you need to find out how many assets are contained in each course, the JS below will assist you.

/* The original "group" */
db.fs.files.group( {
    key: {"_id.course": 1, "_id.org": 1, "_id.run": 1},
    reduce: function(cur, result) { result.count += 1 },
    initial: {count: 0}
} )

/* ..and with category included. */
db.fs.files.group( {
    key: {"_id.course": 1, "_id.org": 1, "_id.run": 1, "_id.category": 1},
    reduce: function(cur, result) { result.count += 1 },
    initial: {count: 0}
} )

var mapFunction = function() {
    var slicer = function(x) { return x.slice(0, x.lastIndexOf("+")) };
    var split_id = null;
    if (typeof this._id === "string")
        split_id = slicer(this._id);
    var key = [ this._id.course, this._id.org, this._id.run, this._id.category, split_id ];
    emit( key, 1 );
};
var reduceFunction = function(key, values) {
    return Array.sum(values);
}
db.fs.files.mapReduce(
    mapFunction,
    reduceFunction,
    {
        out: {inline: 1}
    }
)

/* For debugging the mapper... */
var emit = function(key, value) {
    print("emit");
    print("key: " + key + " value: " + tojson(value));
}

/* To find all courses which aren't the three specified. */
db.fs.files.find({"_id.course": {$nin : ["DemoX", "import_test", "LargeCourse101"]}})

Contentstore

Counting Assets in Each Course