CAD systems allows to save models in several different formats. Among the available saving formats you may find the Parasolid format (.x_t) or the ACIS format (.sat).
ACIS and Parasolid are geometric modeling kernel (i.e. the geometric "engine" of the CAD system) used by several CAD systems, like Solid Edge, Solidworks, Inventor, Ironcad, etc.
Both kernel support hybrid models (i.e. surface and solid models, including multi-body) and in both cases there is no way to know if the file represents an assembly or a single component, by just looking at the file extension.
You can use .x_t, or .sat format to save both single component or an assembly. Moreover, you should be aware of the type of geometry you are saving in the file (wire-frame curves; construction surfaces; solids). In order to be sure about what geometry you are including in the file, don't forget to take a look at the saving option provided by your system.