LiteLLM upload json file is not supported

[kevinkassimo]

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Contribution guide.

Describe the bug
Uploading JSON file when using LiteLLM (for OpenAI models) reports LiteLlm(BaseLlm) does not support this content part.

To Reproduce
Steps to reproduce the behavior:

1. Create a random agent following ADK docs, run adk web
2. Click the attachment button, and select a JSON file
3. Click send. See the error LiteLlm(BaseLlm) does not support this content part.

Expected behavior
Upload should be successful

Screenshots

Desktop (please complete the following information):

• OS: macOS
• Python version(python -V): 3.11
• ADK version(pip show google-adk): 1.17.0

Model Information:

• Are you using LiteLLM: Yes
• Which model is being used(e.g. gemini-2.5-pro): openai/gpt-5

Additional context
N/A

[hangfei]

Fixed now. Please re-open if it's not.