File format not supported message displayed when dropping a non image file over the editor area

STEPS TO REPRODUCE

1. Edit a page in-place or using WYSIWYG
2. Over editor area, drag and drop a non image file (e.g. a .txt file)

EXPECTED RESULTS

A green box is displayed containing the text:

File successfully uploaded.

ACTUAL RESULTS
On top of the successful upload message, a blue box is displayed, containing the following text:

This file format is not supported. You can try with one of the supported formats: PNG, JPEG, GIF.

The issue reproduces also on XWiki 14.1, 14.0, 13.10.3 and 13.10.2, but could not be reproduced on XWiki 13.10.1.