You can launch file picker activity by clicking File->Open from action bar. You can open files from your device, or ftp and cloud storages. Current supported cloud providers are Google Drive, Dropbox and OneDrive.
Only possible text file and directories will be listed in the file picker, other file types will be ignored from the file list. If you want to list all file types, please disable file type filter by clicking "funnel" icon from action bar.
Save File/Save File As
You can click File->Save from action bar to save your modifications. The save menu item will only be enabled when there is modification in current active file.
You can click File->Save As from action bar to save your file to another place with a new name. The name should be unique in target folder.
Please be noticed that on Android 4.4(Kitkat), the external SD card is read-only to third party applications. If you cannot save the modification to external SD card, please save it to internal storage instead.
After Android 5.0(Lollipop), you need to grant QuickEdit write permission to external SD card to make it writable.