Download .vsix von ShadowCube264

Simple browser extension adding a page action to download .vsix files from the Visual Studio Marketplace. The activeTab permission is required to parse the URL and page HTML, for generating a correct download link.

Support may be added for downloading older versions of extensions in the future; however, this may be limited to the past 5 versions, until I can find a better method of determining versions.

If this stops working for any reason, please make an issue! I won't use this too often myself, and others will certainly find errors before I do.