GODRIVER-809 Fix panic while calling existing GridFS data

Fix for: https://jira.mongodb.org/browse/GODRIVER-809

Author: tejzpr

mongo/gridfs/bucket.go

@@ -17,6 +17,7 @@ import (
 	"time"
 
 	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/bson/bsontype"
 	"go.mongodb.org/mongo-driver/bson/primitive"
 	"go.mongodb.org/mongo-driver/mongo"
 	"go.mongodb.org/mongo-driver/mongo/options"
@@ -346,7 +347,14 @@ func (b *Bucket) openDownloadStream(filter interface{}, opts ...*options.FindOpt
 		return nil, err
 	}
 
-	fileLen := fileLenElem.Int64()
+	var fileLen int64
+	switch fileLenElem.Type {
+	case bsontype.Int32:
+		fileLen = int64(fileLenElem.Int32())
+	default:
+		fileLen = fileLenElem.Int64()
+	}
+	
 	if fileLen == 0 {
 		return newDownloadStream(nil, b.chunkSize, 0), nil
 	}