What?

Dynamically computes the file input accept attribute based on WordPress's allowedMimeTypes (from the upload_mimes filter) to prevent users from selecting unsupported file types.

Why?

Fixes #73558

Users could select HEIC files for upload even when the server doesn't support them (lacks Imagick with HEIC support), leading to confusing upload failures. The file picker would show all image types as selectable, but HEIC uploads would fail.

How?

Changes:

1. MediaPlaceholder: Retrieves allowedMimeTypes from editor settings and computes specific MIME type list for the accept attribute
2. MediaReplaceFlow: Applies same logic for media replacement flow
3. Image Block: Removes hardcoded accept="image/*" to allow dynamic computation based on server capabilities
4. Fallback: Falls back to wildcard (e.g., "image/*") when allowedMimeTypes is unavailable

Implementation Details:

• Uses WordPress's existing get_allowed_mime_types() which applies the upload_mimes filter
• This data is already exposed via getSettings().allowedMimeTypes in the block editor
• Dynamically builds the accept attribute by filtering MIME types based on allowedTypes prop
• Example: When HEIC is not supported, accept="image/jpeg,image/png,image/gif,image/webp,image/avif" instead of accept="image/*"

Testing Instructions

Setup

1. Apply this filter to simulate a server without HEIC support:

add_filter( 'upload_mimes', function( $mimes ) {
    unset( $mimes['heic'] );
    return $mimes;
} );

Test

2. Create a new post
3. Add an Image block
4. Click "Upload"
5. Expected: HEIC files should be grayed out/not selectable in the file picker (like how .mov files are grayed out when uploading images)
6. Before: All image files including HEIC were selectable, but upload would fail
7. After: Only server-supported image types are selectable
8. Also, test the other main image-based blocks: Gallery, Cover, and Media & Text

Verify in DevTools

9. Inspect the <input type="file"> element
10. Before: accept="image/*"
11. After: accept="image/jpeg,image/png,image/gif,image/webp,image/avif,..." (without image/heic)

Screencast

Before this PR:

After this PR:

Note

As mentioned in the original issue, this only affects the file picker dialog. Drag-and-drop will still allow selecting HEIC files, but they will fail to upload with an appropriate error message (existing WordPress behavior).