Steps to replicate the issue (include links if applicable):
The relevant code is:
[[File:James Webb Space Telescope Mirror37.jpg|thumb|center|px|caption]]
What happens?:
Linter does not detect "px" as an invalid second caption.
What should have happened instead?:
Linter should detect "px" as an invalid second caption.
Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
Other information (browser name/version, screenshots, etc.):
When working on this bug, please make use of the wide array of "px"-related testcases that should have been created for T215999, T276675, T15500, T207032, T53628, and possibly others.
This is a subtask of T274382.