When working with astronomical data, metadata is also important. A general-purpose file format for transmission, processing and archiving large datasets should facilitate, among other things, both efficient processing of bulk data and encoding of rich semantic metadata. When choosing a format for a particular purpose sometimes no existing format satisfies both these requirements adequately, but combining one data-efficient and one metadata-rich format can be made to do so. This paper discusses considerations for designing such hybrid data/metadata formats, and reviews some examples such as VOParquet, FITS-plus and ECSV. We focus on tabular data, but some of the considerations may apply to other datatypes such as arrays as well.