diff --git a/mirai-core-api/src/commonMain/kotlin/utils/ExternalResource.kt b/mirai-core-api/src/commonMain/kotlin/utils/ExternalResource.kt
index 83e7447d3..7cffc7c5c 100644
--- a/mirai-core-api/src/commonMain/kotlin/utils/ExternalResource.kt
+++ b/mirai-core-api/src/commonMain/kotlin/utils/ExternalResource.kt
@@ -61,9 +61,10 @@ public interface ExternalResource : Closeable {
      * 文件格式，如 "png", "amr". 当无法自动识别格式时为 [DEFAULT_FORMAT_NAME].
      *
      * 默认会从文件头识别, 支持的文件类型:
-     * png, jpg, gif, tif, bmp, wav, amr, silk
+     * png, jpg, gif, tif, bmp, amr, silk
      *
      * @see net.mamoe.mirai.utils.getFileType
+     * @see net.mamoe.mirai.utils.FILE_TYPES
      * @see DEFAULT_FORMAT_NAME
      */
     public val formatName: String
@@ -88,7 +89,7 @@ public interface ExternalResource : Closeable {
 
     public companion object {
         /**
-         * 在无法识别文件格式时使用的默认格式名.
+         * 在无法识别文件格式时使用的默认格式名. "mirai".
          *
          * @see ExternalResource.formatName
          */
@@ -265,7 +266,7 @@ public interface ExternalResource : Closeable {
 
 
 private fun InputStream.detectFileTypeAndClose(): String? {
-    val buffer = ByteArray(10)
+    val buffer = ByteArray(COUNT_BYTES_USED_FOR_DETECTING_FILE_TYPE)
     return use {
         kotlin.runCatching { it.read(buffer) }.onFailure { return null }
         getFileType(buffer)
@@ -320,7 +321,7 @@ internal class ExternalResourceImplByByteArray(
     override val size: Long = data.size.toLong()
     override val md5: ByteArray by lazy { data.md5() }
     override val formatName: String by lazy {
-        formatName ?: getFileType(data.copyOf(8)).orEmpty()
+        formatName ?: getFileType(data.copyOf(COUNT_BYTES_USED_FOR_DETECTING_FILE_TYPE)).orEmpty()
     }
 
     override fun inputStream(): InputStream = data.inputStream()
diff --git a/mirai-core-utils/src/commonMain/kotlin/Files.kt b/mirai-core-utils/src/commonMain/kotlin/Files.kt
index 70d588e83..3b453dbd3 100644
--- a/mirai-core-utils/src/commonMain/kotlin/Files.kt
+++ b/mirai-core-utils/src/commonMain/kotlin/Files.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2020 Mamoe Technologies and contributors.
+ * Copyright 2019-2021 Mamoe Technologies and contributors.
  *
  *  此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
  *  Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
@@ -21,13 +21,18 @@ public val FILE_TYPES: MutableMap<String, String> = mutableMapOf(
     "47494638" to "gif",
     "49492A00" to "tif",
     "424D" to "bmp",
-    "57415645" to "wav",
 
+    // "57415645" to "wav", // server doesn't support
 
     "2321414D52" to "amr",
     "02232153494C4B5F5633" to "silk",
 )
 
+/**
+ * 在 [getFileType] 需要的 [ByteArray] 长度
+ */
+public val COUNT_BYTES_USED_FOR_DETECTING_FILE_TYPE: Int get() = FILE_TYPES.maxOf { it.key.length / 2 }
+
 /*
 
         startsWith("FFD8") -> "jpg"
@@ -40,7 +45,10 @@ public val FILE_TYPES: MutableMap<String, String> = mutableMapOf(
  * 根据文件头获取文件类型
  */
 public fun getFileType(fileHeader: ByteArray): String? {
-    val hex = fileHeader.toUHexString("")
+    val hex = fileHeader.toUHexString(
+        "",
+        length = COUNT_BYTES_USED_FOR_DETECTING_FILE_TYPE.coerceAtMost(fileHeader.size)
+    )
     FILE_TYPES.forEach { (k, v) ->
         if (hex.startsWith(k)) {
             return v