GridFS uses the file name as an index

I am currently working on a "simple" mongoDB photosystem using a replica set and GridFS.

The principle is simple, I put a lot of photos using GridFS, the client knows the file name and from the file name I can get the file.

Is GridFS using file name as indexes? Hopefully, yes, I could not find it in any official document.

My statistics:

     {
        "ns" : "photos.socialphotos.files",
        "count" : 758086,
        "size" : 168295128,
        "avgObjSize" : 222.00004748801587,
        "storageSize" : 220647424,
        "numExtents" : 15,
        "nindexes" : 2,
        "lastExtentSize" : 43311104,
        "paddingFactor" : 1,
        "flags" : 1,
        "totalIndexSize" : 125084624,
        "indexSizes" : {
            "_id_" : 22925504,
            "filename_1_uploadDate_1" : 102159120
        },
        "ok" : 1
    }

EDIT : According to the reIndex () collection, I won 30 Go, but it's still too tall.

My indices:

{
    "v" : 1,
    "key" : {
        "_id" : 1
    },
    "ns" : "photos.socialphotos.files",
    "name" : "_id_"
},
{
    "v" : 1,
    "key" : {
        "filename" : 1,
        "uploadDate" : 1
    },
    "ns" : "photos.socialphotos.files",
    "name" : "filename_1_uploadDate_1"
}

Index Size:

"keysPerIndex" : {
    "photos.socialphotos.files.$_id_" : 758086,
    "photos.socialphotos.files.$filename_1_uploadDate_1" : 758086
}

I never use _id_, because I do not store it, is it right to delete it? The index size is 125084624, which means that I should have almost all of my photos in RAM, which is a bit strange?

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