* in IDE:
# open `/modules/custom/units/widgets/widgets_config.php` file for editing
# add `*.svg` to allowed file extensions in `Image` field declaration and save changes
* in Admin Console:
# go to {nav Custom > Widgets} section
# add 1 record with JPG file (square image with dimensions larger than 120x120) uploaded in `Image` field
# add 1 record with SVG file (square image with dimensions larger than 120x120) uploaded in `Image` field
# confirm, that for both records image thumbnail is shown and it's dimensions are 120x120
# open record with JPG file uploaded
# confirm, that thumbnail in `Image` field is displayed
# close record
# open record with SVG file uploaded
# confirm, that thumbnail in `Image` field isn't displayed (as if non-image was uploaded)
# close record