Always include form value's content-type (#391)

* always include form value's content-type

* keep existing behavior for application/json

index.d.ts

@@ -45,7 +45,7 @@ export interface SavedMultipartFile extends MultipartFile {
 export interface MultipartValue<T = unknown> {
   value: T;
   fieldname: string;
-  mimetype?: string;
+  mimetype: string;
   encoding: string;
   fieldnameTruncated: boolean;
   valueTruncated: boolean;

index.js

@@ -372,8 +372,6 @@ function fastifyMultipart (fastify, options, done) {
     request.pipe(bb)
 
     function onField (name, fieldValue, fieldnameTruncated, valueTruncated, encoding, contentType) {
-      let mimetype
-
       // don't overwrite prototypes
       if (getDescriptor(Object.prototype, name)) {
         onError(new PrototypeViolationError())
@@ -390,7 +388,7 @@ function fastifyMultipart (fastify, options, done) {
 
         try {
           fieldValue = secureJSON.parse(fieldValue)
-          mimetype = 'application/json'
+          contentType = 'application/json'
         } catch (e) {
           onError(new InvalidJSONFieldError())
           return
@@ -399,7 +397,7 @@ function fastifyMultipart (fastify, options, done) {
 
       const value = {
         fieldname: name,
-        mimetype,
+        mimetype: contentType,
         encoding,
         value: fieldValue,
         fieldnameTruncated,

test/multipart-json.test.js

@@ -60,7 +60,7 @@ test('should not parse JSON fields forms if no content-type is set', function (t
   fastify.post('/', async function (req, reply) {
     for await (const part of req.parts()) {
       t.notOk(part.filename)
-      t.notOk(part.mimetype)
+      t.equal(part.mimetype, 'text/plain')
       t.type(part.value, 'string')
     }
 
@@ -93,6 +93,50 @@ test('should not parse JSON fields forms if no content-type is set', function (t
   })
 })
 
+test('should not parse JSON fields forms if non-json content-type is set', function (t) {
+  t.plan(5)
+
+  const fastify = Fastify()
+  t.teardown(fastify.close.bind(fastify))
+
+  fastify.register(multipart)
+
+  fastify.post('/', async function (req, reply) {
+    for await (const part of req.parts()) {
+      t.notOk(part.filename)
+      t.equal(part.mimetype, 'text/css')
+      t.type(part.value, 'string')
+    }
+
+    reply.code(200).send()
+  })
+
+  fastify.listen({ port: 0 }, async function () {
+    // request
+    const form = new FormData()
+    const opts = {
+      protocol: 'http:',
+      hostname: 'localhost',
+      port: fastify.server.address().port,
+      path: '/',
+      headers: form.getHeaders(),
+      method: 'POST'
+    }
+
+    const req = http.request(opts, res => {
+      t.equal(res.statusCode, 200)
+      res.resume()
+      res.on('end', () => {
+        t.pass('res ended successfully')
+      })
+    })
+
+    form.append('css', 'body { width: 100% }', { contentType: 'text/css' })
+
+    form.pipe(req)
+  })
+})
+
 test('should throw error when parsing JSON fields failed', function (t) {
   t.plan(2)