Fixed upload detection and page compatibility issues.

The extension now ignores checkboxes, text fields, links, close buttons, and other normal page controls. The upload chooser only appears for real file upload fields and clearly marked upload zones.

Also fixed stray clipboard UI, improved support for document-only upload fields, corrected image file extensions, and reduced duplicate clipboard watcher activity.

Fixed a compatibility issue where the extension could interfere with complex web apps such as Gmail. The upload chooser is now created only when an upload/select-file area is clicked.