SQLite database
application/vnd.sqlite3
A whole relational database in one file, and quite possibly the most widely deployed file format on earth — SQLite ships inside every browser, phone OS, and most desktop apps. The registered type is application/vnd.sqlite3; the unregistered application/x-sqlite3 predates it and still circulates. The file format is a model of discipline: a fixed 100-byte header that opens with the ASCII string SQLite format 3 followed by a NUL, and a 4-byte application_id at offset 68 that lets derived formats (GeoPackage, for one) brand their databases while staying valid SQLite. The format is formally documented and the project promises backward compatibility through 2050.
.db, .sqlite, and .sqlite3 all circulate; the bytes are identical.
Magic bytes
53 51 4c 69 74 65 20 66 6f 72 6d 61 74 20 33 · "SQLite format 3" · offset 0
Per SQLite Database File Format — header string · test a file against it.
Defined by SQLite Database File Format · IANA registration. Registry facts from the IANA media-types registry via mime-db.