Prevent using a .json format to bypass authorized fetch mode

Patches an issue with authorized fetch mode being able to be bypassed by using a .json format.

Yes, I cherry picked from this Akkoma commit to change this one line.

Checklist

• Adding a changelog: In the changelog.d directory, create a file named <code>.<type>.

  <code> can be anything, but we recommend using a more or less unique identifier to avoid collisions, such as the branch name.

  <type> can be add, remove, fix, security or skip. skip is only used if there is no user-visible change in the MR (for example, only editing comments in the code). Otherwise, choose a type that corresponds to your change.

  In the file, write the changelog entry. For example, if an MR adds group functionality, we can create a file named group.add and write Add group functionality in it.

  If one changelog entry is not enough, you may add more. But that might mean you can split it into two MRs. Only use more than one changelog entry if you really need to (for example, when one change in the code fix two different bugs, or when refactoring).