Modern data lakes have become essential for storing, managing, and analyzing massive amounts of heterogeneous data. As production data increasingly exhibits multimodal storage characteristics and multi-purpose access patterns, efficient management of such complexities becomes critical. However, current hybrid storage system-based data lakes face persistent challenges, including synchronization overhead, data correlation disruption, and escalating storage costs due to the involvement of multiple underlying storage systems. While columnar storage, central to data lakes, addresses hybrid-system inefficiencies, it struggles with the complexities of multimodal data storage and multi-purpose access. To tackle these challenges, we analyze access patterns across various scenarios and assess the issues in storing multimodal data. Based on these insights, we propose FlatStor, a FlatBuffers-based columnar Storage format with embedded indexing. It supports point access through indexing and handles multimodal data by vertically partitioning and treating each modality as a byte stream for storage. It also applies FSST compression, reducing storage overhead significantly. Benchmark evaluations reveal that FlatStor reduces the access latency by 99.6% and the storage overhead by 91.3% compared to Parquet in inference workloads. Furthermore, FlatStor outperforms LanceV2 with a 41.3% latency improvement, maintaining minimal additional overhead