Image-file extension filtering is case sensitive

[Felipe G. M. Maia]

When searching your file system for an image file to create a bootable device (CD, DVD, pendrive, etc), the software offers 2 extensions filter to show the list of files available: "iso" and "img". Since it is a case sensitive filter, files with an "ISO" or "IMG" extension won't be shown and the user might think there is a problem with the software.

2 Suggestions:

- Make the filter case-insensitive.
- Give two filtering options: one for all types of images supported (iso and img) and a second option for all types of files (no filtering).

My system info:

- Package: usb-creator-gtk Version: 0.3.7
- Ubuntu 20.04.1 LTS (Focal Fossa)
- GNOME version 3.36.3