Mongo.GridFs.Bucket.find_one

You're seeing just the function find_one, go back to Mongo.GridFs.Bucket module for more information.
Link to this function

find_one(bucket, file_id)

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Specs

find_one(t(), String.t()) :: BSON.document() | nil
find_one(t(), BSON.ObjectId.t()) :: BSON.document() | nil

Finds one file document by file_id specified either as a string or BSON.ObjectId.