🐛 bug: Fix Content-Type comparison in Is() (#3537)

* fix: improve Is content type matching

* Update ctx.go

---------

Co-authored-by: RW <rene@gofiber.io>

Author: gaby

ctx.go

@@ -877,10 +877,12 @@ func (c *Ctx) Is(extension string) bool {
 		return false
 	}
 
-	return strings.HasPrefix(
-		utils.TrimLeft(c.app.getString(c.fasthttp.Request.Header.ContentType()), ' '),
-		extensionHeader,
-	)
+	ct := c.app.getString(c.fasthttp.Request.Header.ContentType())
+	if i := strings.IndexByte(ct, ';'); i != -1 {
+		ct = ct[:i]
+	}
+	ct = utils.Trim(ct, ' ')
+	return utils.EqualFold(ct, extensionHeader)
 }
 
 // JSON converts any interface or string to JSON.

ctx_test.go

@@ -2091,6 +2091,16 @@ func Test_Ctx_Is(t *testing.T) {
 	utils.AssertEqual(t, false, c.Is("html"))
 	utils.AssertEqual(t, true, c.Is("txt"))
 	utils.AssertEqual(t, true, c.Is(".txt"))
+
+	// case-insensitive and trimmed
+	c.Request().Header.Set(HeaderContentType, "APPLICATION/JSON; charset=utf-8")
+	utils.AssertEqual(t, true, c.Is("json"))
+	utils.AssertEqual(t, true, c.Is(".json"))
+
+	// mismatched subtype should not match
+	c.Request().Header.Set(HeaderContentType, "application/json+xml")
+	utils.AssertEqual(t, false, c.Is("json"))
+	utils.AssertEqual(t, false, c.Is(".json"))
 }
 
 // go test -v -run=^$ -bench=Benchmark_Ctx_Is -benchmem -count=4