Respect forms with enctype set for view transitions

.changeset/rude-geckos-rush.md

@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Respect forms with enctype set for view transitions

packages/astro/src/transitions/router.ts

@@ -462,7 +462,25 @@ async function transition(
 		const init: RequestInit = {};
 		if (preparationEvent.formData) {
 			init.method = 'POST';
-			init.body = preparationEvent.formData;
+			// Form elements without enctype explicitly set default to application/x-www-form-urlencoded.
+			// In order to maintain compatibility with Astro 4.x, we need to check the value of enctype
+			// on the attributes property rather than accessing .enctype directly. Astro 5.x may
+			// introduce defaulting to application/x-www-form-urlencoded as a breaking change, and then
+			// we can access .enctype directly.
+			//
+			// Note: getNamedItem can return null in real life, even if TypeScript doesn't think so, hence
+			// the ?.
+			const form =
+				preparationEvent.sourceElement instanceof HTMLFormElement
+					? preparationEvent.sourceElement
+					: preparationEvent.sourceElement instanceof HTMLElement &&
+					    'form' in preparationEvent.sourceElement
+					  ? (preparationEvent.sourceElement.form as HTMLFormElement)
+					  : preparationEvent.sourceElement?.closest('form');
+			init.body =
+				form?.attributes.getNamedItem('enctype')?.value === 'application/x-www-form-urlencoded'
+					? new URLSearchParams(preparationEvent.formData as any)
+					: preparationEvent.formData;
 		}
 		const response = await fetchHTML(href, init);
 		// If there is a problem fetching the new page, just do an MPA navigation to it.