add FilePart implement for ContentPart to support chat with file

[geekr-dev]

… Gemini

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[geekr-dev]

sorry, the func (FileContent) IsPart() {} method should be isPart with the first letter lowercase, and I hope you can change the method isPart in ContentPart interface to be public, then we can implement our own XXXPart in our logic code

[ImPedro29]

Nice PR! Lets make changes and approve it