Steps to reproduce the issue

Add a custom field of type Media. There is not a way to add alt text as there is in, for example JCE's Image Manager.
Run WAVE toolbar or other accessibility audit to see error.

Expected result

When creating the custom field, after selecting type = Media, include an option to add alt text, possibly after the Default Value option field.

Actual result

Images are displayed without alt text and are flagged as errors in accessibility audits.

System information (as much as possible)

Joomla 3.7.x and higher

Additional comments

Using alt attributes on img elements will allow people using screen readers to extract meaning from images.