Users can drag and drop files from Files and put them into other iOS applications. For example, drag a photo into the Photos application to add it to the photo library.
If you don't like drag and drop, you can manage the file in the traditional way.
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You can use traditional select and move operations
Using bookmarks helps to manage files better, how it works is the same on macOS. Selecting a card color on the Browse column will display all the files you have attached that color. To tag a file or folder, select, hold and drag it to the color card you want in the toolbar.
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Attaching color cards helps manage more effectively
If you have a 3rd party storage application installed, it will automatically appear on Files at the Locations section. If it does not mean that the application does not support Files. Currently, Files that support Files include Google Drive, Box, Dropbox, OneDrive and Transmit.