Handle missing Content-Type header when accessing remote media (#11200)

* add code to handle missing content-type header and a test to verify that it works

* add handling for missing content-type in the /upload endpoint as well

* slightly refactor test code to put private method in approriate place

* handle possible null value for content-type when pulling from the local db

* add changelog

* refactor test and add code to handle missing content-type in cached remote media

* requested changes

* Update changelog.d/11200.bugfix

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>