VSX Extensions: Extension readme preview is not rendered if installed from VSIX file

[safisa]

Bug Description:

Hi,
When installing a VSIX file from the extensions view 'install from VSIX' command, the readme file preview of the extension is not rendered when selecting the extension in the installed extensions list.

Steps to Reproduce:

1. create a new vscode extension with a readme file, and package it as a vsix file.
2. install it from the install from vsix command
3. you should see it listed under the installed extensions list
4. select it to see its readme preview as any other extensions installed from open-vsx
5. no readme file is rendered

Additional Information

• Operating System: mac
• Theia Version: 1.45 and 1.48