[Bug] Gallery isn't exported in HTML and Markdown

### Bug Description

Images in a gallery aren't found when exporting as HTML or Markdown.

### How to Reproduce

1. Create a gallery with images.
2. Export as HTML or Markdown.

### Expected Behavior

Should show a list of images. I know HTML (without CSS) and Markdown don't have support for galleries, grids, staggered views etc but regardless images should be there.

### Operating System

Linux

### AppFlowy Version(s)

v0.6.7

### Screenshots

_No response_

### Additional Context

_No response_