.JET File

File extension jet is associated with the Microsoft JET database engine used by Microsoft Access and Microsoft SQL Server a relational database management software.

A .jet file stores database which seems to be just renamed MDB database file.

Files with jet extension can be most often found as databases in Microsoft JET database file format.

How to open:

To open .jet database in Access rename jet extension to mdb.

How to convert:

Considering these are basically MS Access databases, you can convert them as such.