Copied table from Excel is pasted as raw HTML

### To Reproduce

Steps to reproduce the behavior:
1. Copy table from [example_table.xlsx](https://github.com/github/issues/files/11222266/example_table.xlsx)

2. Open Google Chrome and navigated to Web UI on GHES 3.8 instance (This issue is reproduced in GitHub.com as well)

3. Paste it on a issue comment. It is pasted as raw HTML.
```

<!--table
{mso-displayed-decimal-separator:"\.";
mso-displayed-thousand-separator:"\,";}
@page
{margin:.75in .7in .75in .7in;
mso-header-margin:.3in;
mso-footer-margin:.3in;}
.font5
{color:windowtext;
font-size:6.0pt;
font-weight:400;
font-style:normal;
text-decoration:none;
font-family:游ゴシック;
mso-generic-font-family:auto;
mso-font-charset:128;}
tr
{mso-height-source:auto;
mso-ruby-visibility:none;}
col
{mso-width-source:auto;
mso-ruby-visibility:none;}
br
{mso-data-placement:same-cell;}
td
{padding-top:1px;
padding-right:1px;
padding-left:1px;
mso-ignore:padding;
color:black;
font-size:12.0pt;
font-weight:400;
font-style:normal;
text-decoration:none;
font-family:游ゴシック;
mso-generic-font-family:auto;
mso-font-charset:128;
mso-number-format:General;
text-align:general;
vertical-align:middle;
border:none;
mso-background-source:auto;
mso-pattern:auto;
mso-protection:locked visible;
white-space:nowrap;
mso-rotate:0;}
ruby
{ruby-align:left;}
rt
{color:windowtext;
font-size:6.0pt;
font-weight:400;
font-style:normal;
text-decoration:none;
font-family:游ゴシック;
mso-generic-font-family:auto;
mso-font-charset:128;
mso-char-type:katakana;
display:none;}
-->

a | b
-- | --
1 | 2

```

**Expected behavior**
When I pasted the same excel table in Safari, it was formatted as Markdown style.

```
a | b
-- | --
1 | 2
```

**Screenshots**
Copy and paste table from Excel results in both raw HTML and Markdown. It depends on browser

Raw HTML in Chrome

Markdown in Safari

まず、添付された example_table.xlsx と提供された手順を使い、web UI 上の Chrome と Safari で Excel テーブルの貼り付けを再現します。この問題に関係するブラウザのクリップボード処理と貼り付け処理を追跡します。Chrome が生の Excel HTML を露出させるのではなく、Safari と同じ Markdown 形式のテーブルを生成すれば完了です。

索引モデルが issue の本文から書いたものです。