QMimeDatabase will use a system copy of MIME data if one is present (usually Linux) and an internal copy if one is not.
According to the docs, the built-in copy comes from FreeDesktop shared-mime-info. For Qt 6.5.1 this was release 2.2. This lists "application/fits" as the primary type with alias "image/fits".
Release 2.1 of this set does list "image/fits". This was last in Qt 6.3.
Docs notwithstanding, Qt 6.7 switched to a MIME database from Apache Tika . This lists "application/fits" with no aliases.
Is it possible that the "image/fits" return came from a system database or Qt 6.3 or earlier? My Ubuntu 24.04 has the 2.4 release of the FreeDesktop data.