Adding files is a feature of the paid version of Mongo Management Studio.
When you open a tab with a GridFS collection, then you can find the button "Add file(s)" in the toolbar. With clicking on this button a popup opens, where you can either store files by using drag and drop or by clicking the "Select files" to select files from the file system. After selecting the files, they are displayed in a table with the possibility to delete them again (trash icon). Click on the button "Save" to save the files in the collection.
This functionality may take some time depending on the file size.