Description of the need

This came from reading several other issues about various problems with image uploads, and realizing that we do not have a dedicated form API element for images. We currently have only file and file_managed.

Images usually need different validation criteria than generic files, including * image type validation * image dimension validation

In most places in core, these are added manually to every file upload field used only for images. A new image form API element would make it easier for core developers change all image API elements at once. For example, adding a file type validation that used a site-wide set of allowed image types: * https://github.com/backdrop/backdrop-issues/issues/4937 and * https://github.com/backdrop/backdrop-issues/issues/4998

There are also other image-only upload locations in core, such as the site logo and favicon (and there may be others).

Proposed solution

Add a new image form api element, that is identical to file but with additional validators.

Alternatives that have been considered

Helper functions that could be used to determine a default set of allowed file types, based on the operating system and modules.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes a new form API element specifically for images, that will natively support all image types supported by your system.

GitHub Issue #: 
5946