Improves extension validation for teleporter.php

[rdwebdesign]

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• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes.
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What does this PR aim to accomplish?:

Fix #1455.

The original code assumed that the file extension ".tar.gz" was always after the first dot.

This way, it could accept invalid filenames like:
filename.tar.gz.php

or it could ignore valid filenames like:
file.name.with.dots.tar.gz

How does this PR accomplish the above?:

Changing the code to look for the extension at the end of the filename.

What documentation changes (if any) are needed to support this PR?:

none.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-14-web-v5-11-and-core-v5-9-released/53529/1