[BUGFIX] Selection of non image files in element browser

This patch fixes the condition that checks if an image is selectable
in the "plain" image mode in the RTE.

This fixes a regression that was introduced with #55765.

Resolves: #55939
Releases: 6.2
Change-Id: I92ed00406dcfdba6decb78e2d027f2aa6424ba1b
Reviewed-on: https://review.typo3.org/27585
Reviewed-by: Markus Klein
Tested-by: Markus Klein