Fix #2442 by adding webmanifest extension to content-type map (#19661)

Author: analogrelay

src/Middleware/StaticFiles/src/FileExtensionContentTypeProvider.cs

@@ -343,6 +343,7 @@ public FileExtensionContentTypeProvider()
                 { ".wcm", "application/vnd.ms-works" },
                 { ".wdb", "application/vnd.ms-works" },
                 { ".webm", "video/webm" },
+                { ".webmanifest", "application/manifest+json" }, // https://w3c.github.io/manifest/#media-type-registration
                 { ".webp", "image/webp" },
                 { ".wks", "application/vnd.ms-works" },
                 { ".wm", "video/x-ms-wm" },