Content has no alternative text

Document features such as images cannot be understood by the user of a screen reader unless they have associated alternative text. Therefore, it is necessary to define an alternative text to make sure that the graphic is accessible. 

The accessibility is dependent on the inclusion of a variety of semantic information, such as alternative text for image which is a kind of descriptive metadata. Alternative text is used to describe graphics and their contexts, and its primary purpose is to describe the graphic to visitors who are unable to see them. 

When including alternative text, you ensure that all users can appreciate the content. 

Image checks in Word 2016.

When writing alternative text, strive to be informative and remember the shorter, the better. Use complete sentences, and do not include any information that is already present in the caption. 

File extensions: Some images may contain it's file name as alternative text when inserted.
Accessibility Assistant will check for alternative texts ending with image file extensions such as ".png". 
If the user chooses to handle the conflict by clicking "Add alternative text", the Assistant will suggest the current alternative text minus the file name extension. (i.e. if alt text is "car.png", the assistant will suggest "car". 
List of currently supported image file extensions: .jpg, .jpeg, .png, . gif, .tif