Index: src/wp-admin/includes/media.php
===================================================================
--- src/wp-admin/includes/media.php	(revision 47773)
+++ src/wp-admin/includes/media.php	(working copy)
@@ -3182,7 +3182,7 @@
 	<?php if ( 'image' === substr( $post->post_mime_type, 0, 5 ) ) : ?>
 		<p class="attachment-alt-text">
 			<label for="attachment_alt"><strong><?php _e( 'Alternative Text' ); ?></strong></label><br />
-			<input type="text" class="widefat" name="_wp_attachment_image_alt" id="attachment_alt" aria-describedby="alt-text-description" value="<?php echo esc_attr( $alt_text ); ?>" />
+			<textarea class="widefat" name="_wp_attachment_image_alt" id="attachment_alt" aria-describedby="alt-text-description"><?php echo esc_attr( $alt_text ); ?></textarea>
 		</p>
 		<p class="attachment-alt-text-description" id="alt-text-description">
 		<?php
Index: src/wp-includes/media-template.php
===================================================================
--- src/wp-includes/media-template.php	(revision 47773)
+++ src/wp-includes/media-template.php	(working copy)
@@ -454,7 +454,7 @@
 				<# if ( 'image' === data.type ) { #>
 					<span class="setting has-description" data-setting="alt">
 						<label for="attachment-details-two-column-alt-text" class="name"><?php _e( 'Alternative Text' ); ?></label>
-						<input type="text" id="attachment-details-two-column-alt-text" value="{{ data.alt }}" aria-describedby="alt-text-description" {{ maybeReadOnly }} />
+						<textarea id="attachment-details-two-column-alt-text" aria-describedby="alt-text-description" {{ maybeReadOnly }}>{{ data.alt }}</textarea>
 					</span>
 					<p class="description" id="alt-text-description"><?php echo $alt_text_description; ?></p>
 				<# } #>
@@ -655,7 +655,7 @@
 		<# if ( 'image' === data.type ) { #>
 			<span class="setting has-description" data-setting="alt">
 				<label for="attachment-details-alt-text" class="name"><?php _e( 'Alt Text' ); ?></label>
-				<input type="text" id="attachment-details-alt-text" value="{{ data.alt }}" aria-describedby="alt-text-description" {{ maybeReadOnly }} />
+				<textarea id="attachment-details-alt-text" aria-describedby="alt-text-description" {{ maybeReadOnly }}>{{ data.alt }}</textarea>
 			</span>
 			<p class="description" id="alt-text-description"><?php echo $alt_text_description; ?></p>
 		<# } #>
@@ -957,7 +957,7 @@
 
 		<span class="setting alt-text has-description">
 			<label for="embed-image-settings-alt-text" class="name"><?php _e( 'Alternative Text' ); ?></label>
-			<input type="text" id="embed-image-settings-alt-text" data-setting="alt" aria-describedby="alt-text-description" />
+			<textarea id="embed-image-settings-alt-text" data-setting="alt" aria-describedby="alt-text-description"></textarea>
 		</span>
 		<p class="description" id="alt-text-description"><?php echo $alt_text_description; ?></p>
 
@@ -1020,7 +1020,7 @@
 				<div class="column-settings">
 					<span class="setting alt-text has-description">
 						<label for="image-details-alt-text" class="name"><?php _e( 'Alternative Text' ); ?></label>
-						<input type="text" id="image-details-alt-text" data-setting="alt" value="{{ data.model.alt }}" aria-describedby="alt-text-description" />
+						<textarea id="image-details-alt-text" data-setting="alt" aria-describedby="alt-text-description">{{ data.model.alt }}</textarea>
 					</span>
 					<p class="description" id="alt-text-description"><?php echo $alt_text_description; ?></p>