Uppercase file extensions are not recognized when saving

Summary:

When overwriting a file (with save as/save a copy) that has a capitalized extension, Inkscape doesn't recognize it and instead creates filename.SVG.svg.

Steps to reproduce:

• Create a file ending in .SVG.
• open Inkscape
• do File Save As, and click the file to overwrite it
• click the save button
• wait for the overwrite confirmation box
• be confused when it doesn't appear

What happened?

• realize you now have a file ending in .SVG and a new file ending in .SVG.svg

What should have happened?

It should have stomped on the previous .SVG, preserving its case.

Debug info

1.0.2 (e86c870879, 2021-01-15)