[file_selector_web] feat: Support XFile.mimeType

[Gustl22]

Support providing mime type to XFile blob on Web file_selector.

This is useful (as in other contexts of web) to provide the mimeType to e.g. a media player to support blob urls.

Related: flutter/flutter#93379

List which issues are fixed by this PR. You must list at least one issue.

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Code Review

This pull request adds support for mimeType to XFile in the file_selector_web package. The changes include updating the _convertFileToXFile method to extract the MIME type from the web File object and enhancing integration tests to verify this property. The package version is bumped to 0.9.5. I have no feedback to provide as there were no review comments.