Fix Content-Type

[lishaduck]

Description

This PR makes the .pck file use the correct Content-Type.

Closes: #38

---

Type of Change

• 🐛 Bug fix

Checklist

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Fill out this template.
• Log your hours.
• Check that commits follow the Angular commit convention, more or less.
• Ideally, include relevant tests that fail without this PR but pass with it (if applicable).

Tested on

• macOS 14

[lishaduck]

I tried to add a test for compression, but that's done natively, so it can't (readily) be tested from userland. We'll just have to trust Deno with it.