fix(server): keep the response Content-Type when compressing Blob bodies

packages/server/src/plugins/response-compression.test.ts

@@ -670,6 +670,40 @@ describe('responseCompressionHandlerPlugin', () => {
       expect(matched).toBe(true)
       expect(response!.headers.has('content-encoding')).toBe(false)
     })
+
+    it.each([
+      ['text/csv; charset=utf-8', 'text/plain', 'gzip'],
+      ['text/html; charset=utf-8', '', 'gzip'],
+      ['image/png', 'text/plain', null],
+      [[], 'text/plain', null],
+    ])('uses content-type header %j over blob type %j', async (contentType, blobType, encoding) => {
+      const handler = new RPCHandler(os.handler(() => new Blob(['large content'.repeat(100)], { type: blobType })), {
+        plugins: [new ResponseCompressionHandlerPlugin({ threshold: 100 })],
+        routingInterceptors: [async ({ next }) => {
+          const result = await next()
+
+          return result.matched
+            ? { ...result, response: { ...result.response, headers: { ...result.response.headers, 'content-type': contentType } } }
+            : result
+        }],
+      })
+
+      const handle = (acceptEncoding: string) => handler.handle(new Request('http://localhost', {
+        method: 'POST',
+        headers: {
+          'content-type': 'application/json',
+          'accept-encoding': acceptEncoding,
+        },
+        body: JSON.stringify({ json: null }),
+      }))
+
+      const { response } = await handle('gzip')
+      const { response: uncompressed } = await handle('identity')
+
+      expect(response!.headers.get('content-encoding')).toBe(encoding)
+      // Same content-type the adapter sends when the response is not compressed
+      expect(response!.headers.get('content-type')).toBe(uncompressed!.headers.get('content-type'))
+    })
   })
 
   describe('readable stream body', () => {

packages/server/src/plugins/response-compression.ts

@@ -102,9 +102,12 @@ export class ResponseCompressionHandlerPlugin<T extends Context> implements Stan
       }
 
       else if (body instanceof Blob) {
+        // Adapters only fall back to the blob type when no Content-Type header is set
+        const contentType = headers['content-type'] ?? body.type
+
         if (
           (!Number.isFinite(body.size) || body.size >= this.threshold)
-          && this.isCompressibleContentType(body.type, interceptorOptions)
+          && this.isCompressibleContentType(flattenStandardHeader(contentType), interceptorOptions)
         ) {
           const contentDisposition = headers['content-disposition'] ?? generateContentDisposition(
             body instanceof File ? body.name : 'blob',
@@ -118,7 +121,7 @@ export class ResponseCompressionHandlerPlugin<T extends Context> implements Stan
               headers: {
                 ...toCompressedHeaders(headers, encoding),
                 'standard-server': 'file' satisfies StandardBodyHint,
-                'content-type': body.type,
+                'content-type': contentType,
                 'content-disposition': contentDisposition,
               },
             },