Adding files is a feature of the paid version of Mongo Management Studio.
When you open a tab with a GridFS collection, there is the button "Add file(s)" in the toolbar. Click on this button, a window opens, where you can either store the files using drag and drop or by clicking the "Select files" from the file system to select. 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.