### Bug description

I am having troubles by adding extensions from a GitLab repo, which I am developing. The extension seems to be downloaded but it fails when unzipping the zip-file.

### Steps to reproduce

```bash
quarto add https://gitlab.cc-asp.fraunhofer.de/quarto-extensions/mdz/-/archive/main/shorty-main.zip
```

### Expected behavior

The installed extension.

### Actual behavior

An error message is retrieved:

```
Quarto extensions may execute code when documents are rendered. If you do not 
trust the authors of the extension, we recommend that you do not install or 
use the extension.
 ? Do you trust the authors of this extension (Y/n) › Yes
[✓] Downloading
(|) Unzipping
gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
[✓] Unzipping
ERROR: Failed to unzip extension.

gzip: stdin: not in gzip format
tar: Child returned status 1
tar: Error is not recoverable: exiting now
```

### Your environment

- IDE: VScode
- OS: Ubuntu

### Quarto check output

```
[✓] Checking Quarto installation......OK
      Version: 1.2.335
      Path: /opt/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.10.6
      Path: /usr/bin/python3
      Jupyter: 5.1.5
      Kernels: ir, python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.1.2
      Path: /usr/lib/R
      LibPaths:
        - /home/miguel/R/x86_64-pc-linux-gnu-library/4.1
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      rmarkdown: 2.20

[✓] Checking Knitr engine render......OK
```
dragonstyle
2023-06-06T08:37:49Z
kamapu
bug documentation extensions triaged-to
closed
Installing extension from private Gitlab repository
https://github.com/quarto-dev/quarto-cli/issues/5827