avoid duplication of .gz suffix

[GhostCore07]

prevents an issue where when a .gz file is requested it attempts to append an additional .gz suffix prior to checking if the file exists, i.e. the extension becomes .gz.gz

Relevant issues

Contributor checklist

• Provide tests for the changes (unless documentation-only)
• Documented any new features, CLI switches, etc. (if applicable)
  • Server --help output
  • README.md
  • doc/http-server.1 (use the same format as other entries)
• The pull request is being made against the master branch

Maintainer checklist

• Assign a version triage tag
• Approve tests if applicable