feat: attachment open via xdg-open on Linux, mime type detection

- Use xdg-open directly on Linux for opening attachments (fixes 'file type not supported' error)
- Add mime package for comprehensive MIME type detection in compose screen
- Show file size and MIME type for attachments in compose screen
- Add open/preview button for attachments in compose screen

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

LATER.md

@@ -4,10 +4,6 @@ Thread view (group by `References` / `In-Reply-To`)
 
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-Attachment download + open
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 mail-loop.com (anstatt shared inbox).
 
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@@ -21,15 +17,6 @@ create a plan. Avoid downloading big bodies/attachments again.
 
 mailcoach.de
 
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-done?
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-think about that: Maybe we should not mock jmap/imap/smtp. We  have a temproary Stalwart.
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-Just like mocking DB in Django makes no sense.
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 Try Qwen, vscode plugin
@@ -52,12 +39,6 @@ After Try Connection, show some matching icon next to the text.
 
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-Mail edit, attachment:
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-List of attached files should be visible and editable. Show size, and type of file.
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-Make it possible to open/view the file.
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 Test with a Fastmail account