Preconditions: existing content type with textline and image (both required) and media and file (not required).
Steps:
1. Go to Create mode with content type from preconditions
2. Nothing is filled, click Publish - validation messages are show - OK
3. Drag'n'drop an image to the field, validation message for alternative text is shown, but please make a space between ":" and "Alternative" (see attachment)
4. Remove image from field using trash button
5. Click publish - validation message is shown but it's incomplete (see atachment)