The instructions have been checked against version 8.2.7. Some screenshots are from earlier versions, so labels, icons, or layout may differ.
Import from Files
On the app's file list screen, tap the add button and choose the Files import option. Select a supported video or audio file to copy it into the current folder.
Import from another app
Select a file in the source app, open the iOS share sheet, and choose Easy MP3 Converter. The label and position in the share sheet may vary by iOS version and source app.