Repository: syncthing/syncthing (language: go)

Issue:
GUI refers to `/assets/css/customicons.css` which doesn't exist
Syncthing v1.23.1 via https://apt.syncthing.net, Ubuntu 20.04LTS x64. Chromium v110. I'm using Nginx as a proxy to access remotely.

There's a 404 on `/assets/css/customicons.css` when strict MIME type checking is enabled. Other CSS files in that directory seem to load as expected.

```
Refused to apply style from 'https://host.example.com:123/assets/css/customicons.css' because its MIME type ('text/plain') is not a supported stylesheet MIME type, and strict MIME checking is enabled.
```

If I load `customicons.css` at the server with `curl`, I get this (unexpected):

```
$ curl 127.0.0.1:8384/assets/css/customicons.css
404 page not found
```

If I load a known-good CSS from the same directory, I get this (expected):

```
$ curl 127.0.0.1:8384/assets/css/tree.css
/*
// Copyright (C) 2021 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.

*/

.fancytree-container {
    cursor: pointer;
    width: 100%;
}

.fancytree-hide {
    visibility: collapse;
}

/* Node needs to be block, and expander, icon and title
   inline-block to properly wrap unbreakable text. */
.fancytree-node {
    display: block;
    white-space: nowrap;
    /* expander 16px + icon 16px + title padding 8px */
    padding-right: 40px;
}
.fancytree-expander,
.fancytree-icon,
.fancytree-title {
    display: inline-block;
}

.fancytree-expander,
.fancytree-icon {
    margin-top: 4px;
    vertical-align: top;
    width: 16px;
}

.fancytree-childcounter {
    background: #777;
    border-radius: 10px;
    border: 1px solid gray;
    color: #fff;
    font-size: 13px;
    opacity: .75;
    padding: 2px 3px;
    position: relative;
    right: 8px;
    top: -9px;
    user-select: none;
}

.fancytree-title {
    padding-left: 8px;
    white-space: normal;
    word-break: break-all;
}
```

The other CSS files referenced in that directory also seem to work as expected, it's `customicons.css` that throws the error.



Discussion / hints:

Fix patch:
diff --git a/gui/default/index.html b/gui/default/index.html
index b527d98dd69..ac84df04c64 100644
--- a/gui/default/index.html
+++ b/gui/default/index.html
@@ -24,7 +24,6 @@
   <link href="assets/css/tree.css" rel="stylesheet"/>
   <link href="assets/css/overrides.css" rel="stylesheet"/>
   <link href="assets/css/theme.css" rel="stylesheet"/>
-  <link href="assets/css/customicons.css" rel="stylesheet"/>
 </head>
 
 <body>


Test patch: