try fix epub on kindle for some clients

index.js

@@ -232,10 +232,14 @@ router.post('/upload', upload.single('file'), async ctx => {
       return
     }
 
-    const mimetype = ctx.request.file.mimetype
+    let mimetype = ctx.request.file.mimetype
 
     const type = await FileType.fromFile(ctx.request.file.path)
 
+    if (mimetype == "application/octet-stream" && type) {
+      mimetype = type.mime
+    }
+
     if ((!type || !allowedTypes.includes(type.mime)) && !allowedTypes.includes(mimetype)) {
       flash(ctx, {
         message: 'Uploaded file is of an invalid type: ' + ctx.request.file.originalname + ' (' + (type? type.mime : 'unknown mimetype') + ')',