[BUG] - ServerFolders - can't use custom folder icons

[jrporter]

Describe the bug
I am unable to use local files for my custom icons. Irrespective of image format and resolution used, when choosing image files on the customer icon screen, they render as blank/invisible in the icon preview section. After adding, the confirmation message appears that the icon has been added, but it doesn't actually work. It just exists as a blank/invisible icon.

To Reproduce

1. Right click on a server folder
2. Select 'folder settings'
3. Select 'custom icons'
4. Select 'browse file' x 2, navigating to and choosing image files for both the open and closed icon
5. Select 'add'

Expected behavior
I expect my selected images to be used as the server folder icon.

Discord Build (stable, ptb or canary):
Stable

Additional context
Notably, on the custom icons screen, if I type URLs directly in the text boxes rather than clicking the "choose file" button, the icons will render as expected in the preview section of this screen. However, the 'add' button doesn't do anything when clicked, so this is not a viable workaround (and might also be a separate bug?).