Keep support for ICO formats by defaulting to PNG if necessary

[kmcgrady]

📚 Context

Please describe the project or issue background here

• What kind of change does this PR introduce?

  • Bugfix

🧠 Description of Changes

• We introduced an issue in png as default favicon to allow alpha transparency #4272 that made Streamlit decide the best format.
• This fails at "unrecognized formats", namely an ICO format for the page icon
  • An unrecognized format is one that is understood by Pillow but not by imghdr
• It should be noted that any unrecognized formats would fail as well, and we should be more resilient
• The change suggests that if imghdr cannot determine the image, aim for the default mimetype as defined by "auto".

No screenshot needed

🧪 Testing Done

• Added/Updated e2e tests

🌐 References

Does this depend on other work, documents, or tickets?

• Issue: Closes ICO formats fail on set_page_config page icons #4339

---

Contribution License Agreement

By submitting this pull request you agree that all contributions to this project are made under the Apache 2.0 license.