fix(useFetch): doesn't work with formData payload (#2440)

Content-Type should not be set for payload of formData type,  leave alone and let the browser do.

reference: [Automatic Content-Type for FormData uploads](axios/axios@2f4d0b8)

packages/core/useFetch/index.ts

@@ -95,7 +95,6 @@ type Combination = 'overwrite' | 'chain'
 const payloadMapping: Record<string, string> = {
   json: 'application/json',
   text: 'text/plain',
-  formData: 'multipart/form-data',
 }
 
 export interface BeforeFetchContext {
@@ -532,9 +531,9 @@ export function useFetch<T>(url: MaybeComputedRef<string>, ...args: any[]): UseF
         }
 
         const rawPayload = resolveUnref(config.payload)
-        // Set the payload to json type only if it's not provided and a literal object is provided
+        // Set the payload to json type only if it's not provided and a literal object is provided and the object is not `formData`
         // The only case we can deduce the content type and `fetch` can't
-        if (!payloadType && rawPayload && Object.getPrototypeOf(rawPayload) === Object.prototype)
+        if (!payloadType && rawPayload && Object.getPrototypeOf(rawPayload) === Object.prototype && !(rawPayload instanceof FormData))
           config.payloadType = 'json'
 
         return {