fix: file content can't be set when main_type is 'text', 'image', or …

[ShilongLee]

fix attachment bug sending emails

Description

I found that the attachment I sent in the email only contains a single string "r". I later realized that the code did not read the file content, but instead wrote a default "r".

Fixes # (issue)

Has it been tested?

• Development testing done
• Unit or integration test implemented

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have performed a peer-reviewed with team member(s)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

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[ShilongLee]

Now, it can successfully send emails with attachments.