MongoDB Documentation, Release 3.0.4 { "_id" : <ObjectId>, "files_id" : <ObjectId>, "n" : <num>, "data" : <binary> } { "_id" : <ObjectId>, "length" : <num>, "chunkSize" : <num>, "uploadDate" : <timestamp>, "md5" : <hash>, "filename" : <string>, "contentType" : <string>, "aliases" : <string array>, "metadata" : <dataObject>, } files. length files. _id files. chunkSize files. uploadDate The unique ID for this document. The _id is of the data type you chose for the original document. The default type for MongoDB documents is BSON ObjectId . The size of the document in bytes. The size of each chunk. GridFS divides the document into chunks of the size specified here. The default size is 255 kilobytes. Changed in version 2.4.10: The default chunk size changed from 256k to 255k. The date the document was first stored by GridFS. This value has the Date type. Documents in the files collection contain some or all of the following fields. Applications may create additional arbitrary fields: 175 4.4. Data Model Reference A document from the chunks collection contains the following fields: The unique ObjectId of the chunk. The _id of the “parent” document, as specified in the files collection. The sequence number of the chunk. GridFS numbers all chunks, starting with 0. The chunk’s payload as a BSON binary type. The chunks collection uses a compound index on files_id and n , as described in GridFS Index (page 151). Each document in the files collection represents a file in the GridFS store. Consider the following prototype of a document in the files collection: The files Collection chunks. _id chunks. files_id chunks. data chunks. n