Enable Attachment Processing - INVALID_ARGUMENT Error (mimeType message/rfc822)

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[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

**Is your feature request related to a problem? Please describe.**
I am unable to process attachments using the Google ADK. When I attempt to attach a file, I receive the following error in the chat:

`400 INVALID_ARGUMENT. {'error': {'code': 400, 'message': 'Unable to submit request because it has a mimeType parameter with value message/rfc822, which is not supported. Update the mimeType and try again. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/model-reference/gemini', 'status': 'INVALID_ARGUMENT'}}`

This limitation prevents the development of applications that require handling file uploads.

**Describe the solution you'd like**
I would like the Google ADK to support the processing of attachments. This would involve enabling the system to correctly handle different mimeTypes beyond the currently supported ones, specifically including the ability to process files sent as attachments. Additionally, providing clear documentation and potentially helper functions within the ADK to facilitate attachment handling would be highly beneficial.

**Describe alternatives you've considered**
I have explored the error message and the linked documentation, but it primarily focuses on the mimeType of the main message content rather than attachments. I haven't found any clear alternative within the current ADK to directly process file attachments.

**Additional context**
It would be extremely helpful to have a working example demonstrating how to properly attach a file and access its content within the Google ADK. This example should illustrate the correct way to format the request and handle the attachment data on the receiving end. Understanding the supported mimeTypes for attachments and the expected structure of the attachment data in the request is crucial for implementation.