Restrict the support type of plain-text documents for Gemini

[adubovik]

#100 has introduced support of the following plain-text files: txt, html, css, js, ts, csv, md, py, json, xml, rtf.

However, it seems like Gemini has restricted this list to only txt: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/document-understanding#document-requirements.

E.g. gemini-1.5-flash-001 fails with the following error when given .json file:

400 Unable to submit request because it has a mimeType parameter with value application/json, which is not supported. Update the mimeType and try again

1. We need to verify this statement,
2. If true, restrict the list of supported types in the adapter