diff --git a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
index ae486554..ad6decd4 100644
--- a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
+++ b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
@@ -176,7 +176,7 @@ class AttachmentRequest(
                 false
             }
 
-            val canUseOriginal = when {
+            val canUseOriginal: Boolean = when {
                 // WebPを使っていい場合、PNG画像をWebPに変換したい
                 canUseWebP && mimeType == MIME_TYPE_PNG -> false
                 // WebPを使わない場合、入力がWebPなら強制的にPNGかJPEGにする
@@ -219,7 +219,7 @@ class AttachmentRequest(
                         @Suppress("DEPRECATION")
                         Bitmap.CompressFormat.WEBP
                 }
-                return compressToTempFileOpener(MIME_TYPE_WEBP, format, 90)
+                return compressToTempFileOpener("webp", MIME_TYPE_WEBP, format, 90)
             } catch (ex: Throwable) {
                 log.w(ex, "compress to WebP lossy failed.")
                 // 失敗したらJPEG or PNG にフォールバック
@@ -234,12 +234,14 @@ class AttachmentRequest(
             }
             return when (hasAlpha) {
                 true -> compressToTempFileOpener(
+                    "png",
                     MIME_TYPE_PNG,
                     Bitmap.CompressFormat.PNG,
                     100
                 )
 
                 else -> compressToTempFileOpener(
+                    "jpg",
                     MIME_TYPE_JPEG,
                     Bitmap.CompressFormat.JPEG,
                     95
@@ -255,6 +257,7 @@ class AttachmentRequest(
      * 失敗したら例外を投げる
      */
     private fun Bitmap.compressToTempFileOpener(
+        fixExt: String?,
         outMimeType: String,
         format: Bitmap.CompressFormat,
         quality: Int,
@@ -262,7 +265,7 @@ class AttachmentRequest(
         val tempFile = context.generateTempFile("createResizedImageOpener")
         try {
             FileOutputStream(tempFile).use { compress(format, quality, it) }
-            return tempFileOpener(tempFile, outMimeType, isImage = true)
+            return tempFileOpener(tempFile, outMimeType, isImage = true, fixExt = fixExt)
         } catch (ex: Throwable) {
             tempFile.delete()
             throw ex
@@ -362,6 +365,10 @@ class AttachmentRequest(
                     else -> "video/mp4"
                 },
                 isImage = false,
+                fixExt = when (result) {
+                    tempFile -> null
+                    else -> "mp4"
+                },
             )
         } finally {
             if (outFile != resultFile) outFile.delete()
diff --git a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
index 400cdecb..1e21face 100644
--- a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
+++ b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
@@ -177,7 +177,10 @@ class AttachmentUploader(
                 error(safeContext.getString(R.string.mime_type_not_acceptable, opener.mimeType))
             }
 
-            val fileName = fixDocumentName(getDocumentName(safeContext.contentResolver, uri))
+            val fileName = fixDocumentName(
+                getDocumentName(safeContext.contentResolver, uri),
+                fixExt = opener.fixExt,
+            )
             pa.progress = safeContext.getString(R.string.attachment_handling_uploading, 0)
             fun writeProgress(percent: Int) {
                 pa.progress = if (percent < 100) {
@@ -289,7 +292,7 @@ class AttachmentUploader(
         }
     }
 
-    private fun fixDocumentName(s: String): String {
+    private fun fixDocumentName(s: String, fixExt: String?): String {
         val sLength = s.length
         val m = """([^\x20-\x7f])""".asciiPattern().matcher(s)
         m.reset()
@@ -302,7 +305,11 @@ class AttachmentUploader(
             lastEnd = m.end()
         }
         if (lastEnd < sLength) sb.append(s.substring(lastEnd, sLength))
-        return sb.toString()
+        var escaped = sb.toString()
+        if (!fixExt.isNullOrEmpty()) {
+            escaped = """\.[^./\\]*\z""".toRegex().replace(escaped, "") + ".${fixExt}"
+        }
+        return escaped
     }
 
     ///////////////////////////////////////////////////////////////
@@ -339,8 +346,10 @@ class AttachmentUploader(
                     )
                 }
 
-                val fileName =
-                    fixDocumentName(getDocumentName(safeContext.contentResolver, src.uri))
+                val fileName = fixDocumentName(
+                    getDocumentName(safeContext.contentResolver, src.uri),
+                    fixExt = opener.fixExt,
+                )
 
                 if (account.isMisskey) {
                     TootApiResult("custom thumbnail is not supported on misskey account.")
diff --git a/app/src/main/java/jp/juggler/subwaytooter/util/InputStreamOpener.kt b/app/src/main/java/jp/juggler/subwaytooter/util/InputStreamOpener.kt
index e7081d8c..b5025091 100644
--- a/app/src/main/java/jp/juggler/subwaytooter/util/InputStreamOpener.kt
+++ b/app/src/main/java/jp/juggler/subwaytooter/util/InputStreamOpener.kt
@@ -16,6 +16,8 @@ abstract class InputStreamOpener {
     abstract val mimeType: String
     abstract val isImage: Boolean
 
+    open val fixExt :String? = null
+
     @Throws(IOException::class)
     abstract fun open(): InputStream
 
@@ -74,9 +76,11 @@ fun tempFileOpener(
     file: File,
     mimeType: String,
     isImage: Boolean,
+    fixExt:String? = null,
 ) = object : InputStreamOpener() {
     override val mimeType = mimeType
     override val isImage = isImage
+    override val fixExt = fixExt
 
     @Throws(IOException::class)
     override fun open() = FileInputStream(file)