Using the script checkoutextensions.sh or checkoutextensions.cmd respectively for Linux and Windows.
For Windows users, please open your Command Prompt and execute the following command: checkoutextensions.cmd
For Linux users, please open your Terminal and execute the following command: sh checkoutextensions.sh
Please note that after running the script all extension folders in data/ext are connected by symlinks to the extensions in the extensions. In this folder you will find cloned the repositories of all supported TagSpaces extension. This way you can make changes in for e.g. extensions/viewerImage, which will be immediately testable after running the application, because of the symlink.
On application loading TagSpaces is scanning the extension folder (e.g. data/ext) for available extensions. So basically it is searching every sub folder for a bower file. From the bower file TagSpaces is extractiong the id and the name of the extension, which are needed later. Currently on Firefox and Chrome the available extensions are fixed in settings and not resolved at runtime.
In the most extensions like [viewerImage] or [viewerMD] the extension.js is creating dynamically a new IFRAME elements which loads a file called index.html, where the image or markdown content is displayed or manipulated.