Add file signature detection

It would be really cool to have Amaze check the file sigature

There are at least two use cases:

• files with no extension. It could automatically determine the filetype instead of showing the "open as " dialog
• Misnamed files. For example if a zip file is reported as jpg by the file sigature check, the already implemented popup dialog "open as" could show up and let the user choose the right format. It would also allow to show thumbnails for files not detected as media files by extension

Thanks for consideration

Start by locating the existing "open as" popup and the extension-based file-type and thumbnail handling mentioned in the issue. Trace how those paths identify files, then define completion as signature checks handling files without extensions and misnamed files while integrating with the existing chooser and thumbnail behavior.