SQLite Forum

No ".open" option in version 3.7.17 2013-05-20
That test tells you it is not a typical sqlite3 database file. So you've eliminated one possibility out of many (and mostly the only one this forum discusses).

Since you have unix utilities, you might try the "file" command, which (according to its man page) determines file types.

If I rename a Microsoft Access .mdb file to abc.def I get

$ file abc.def
abc.def: Microsoft Access Database

so it sometimes works.