The GetSharePointFile.aspx page is a special page located under the ~/CMSModules/Sharepoint/CMSPages directory. The correct URL to access the page looks like the following:
It downloads the specified file from the SharePoint server and sends it further to the user. Credentials configured in Site Manager -> Settings -> Microsoft SharePoint are used for authentication by the web service. You can control the Content-Disposition HTTP header using the disposition query parameter.
For images, it supports maxsidesize, width and height parameters to control the image size. Kentico CMS cache is used for images, you can configure it in Site Manager -> Settings -> website -> Cache images (minutes).
Because the settings configured in Site Manager -> Settings -> Microsoft SharePoint are automatically used for authentication by the GetSharePointFile.aspx page and since the URL to access the page can be entered manually, it is highly recommended to enter the credentials of a user that is authorized to access only the files you want to display on your website.