File previews unavailable in embed mode

When embed=true or WEB_OPTION_EMBED_ENABLED=true, file previews for types like txt, png, jpeg, etc no longer work. Previewing files is a standard feature on any file browser and this option should not be disabled when in embed mode.

Start by locating the handling of embed=true and WEB_OPTION_EMBED_ENABLED, then trace where previews for txt, png, and jpeg files are gated. Verify that those file types can be previewed when embed mode is enabled, while the existing non-embed behavior remains unchanged.

Written by the indexing model from the issue text.