MongoDB utilizes BSON (Binary JSON) as the primary data format. It is a binary format that stores key/value pairs in a single entity called document. For example, a sample JSON, {"hello":"world"}, becomes \x16\x00\x00\x00\x02hello\x00\x06\x00\x00\x00world\x00\x00 when encoded in BSON.
BSON stores data rather than literals. For instance, if an image is to be part of the document, it will not have to be converted to a base64-encoded string; instead, it will be directly stored as binary data, unlike plain JSON, which will usually represent such data as base64-encoded bytes, but that is obviously not the most efficient way.
Mongoose schemas enable storing binary content in the BSON format via the schema type—buffer. It stores binary content (image, ZIP archive, and so on) up to 16 MB. The reason behind the relatively small storage capacity is to prevent excessive usage of memory and bandwidth during transmission.