A device generates a Network Abstraction Layer (NAL) Unit Map Entry box in a file. The NAL Unit Map Entry box associates NAL units with a tile set that includes two or more tiles of a plurality of tiles into which a current picture is partitioned. A region of the current picture covered by the tile set may be non-rectangular. Additionally, the device generates a tile set group entry in the file. The tile set group entry includes a horizontal offset, a vertical offset, a width, and a height. The horizontal offset specifies a horizontal offset of a top-left pixel of a minimum rectangular region that covers tiles in the tile set. The vertical offset specifies a vertical offset of the top-left pixel of the minimum rectangular region. The width specifies a width of the minimum rectangular region. The height specifies a height of the minimum rectangular region.