Files in Infinit
The first option, known as the custom picker displays only the photos and videos associated with your Camera album in your gallery. You can select any number of files using this picker, but will be limited to that album.
You can also use the default picker on Android, which will give you access to all of the folders on your phone including the albums in your Gallery. However, with this option only one item can be selected at a time.
You can alternate between the type of file picker you would like to use from the Settings screen.
Files outside Infinit
You can use the sharing option from any other app on your phone to select any number of files and share that content through Infinit.