BB 42220: Allow for more file types to be forced-inline.

Firefox allows to open some files in the browser without any
confirmation, but this will result in a disk leak, because the file will
be downloaded to the temporary directory first (and not deleted, in some
cases).
A preference allows PDFs to be opened without being downloaded to disk.
So, we introduce a similar one to do the same for all the files that are
set to be opened automatically in the browser, except svg and html files
to prevent XSS hazards (see BB 43211).