Change the method of get file extension.

[myafer]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix.

⤵️ What is the current behavior?

When the http response's Content-Type is not image and the _resizeimage is not work.

🆕 What is the new behavior (if this is a feature change)?

The new method is get files extension with the file's binary data.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

📝 Links to relevant issues/docs

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[cedvdb]

@renefloor could this be reviewed ? using magic bytes is preferred over a potentially missing extension

[rickdijk]

main has moved since you opened this, and our changes to example/android/settings.gradle and pubspec.yaml left it conflicting. I have merged main into your branch and resolved those conflicts. In both files main had already moved past the versions this pull request set (Kotlin 2.3.20 rather than 1.8.22, and build_runner ^2.14.0 rather than ^2.4.9), so I kept main's lines there. Your image_cache_manager.dart change is untouched apart from one line reflowed by dart format.