ICO format support
A user on Metadata Cleaner's matrix channel asked about metadata in ICO files (the format used by Windows applications for their icons and websites for their favicons).
According to Wikipedia, the format can store PNG images, so in theory metadata could be passed that way.
After some testing with icoutils, when creating an ICO from a PNG containing metadata with the raw option (icotool -c -r source.png -o icon.ico) and extracting it again (icotool -x icon.ico -o extracted.png), the metadata is still present in the extracted PNG. However, the generated ICO appears to be broken. Without the raw option, metadata is stripped, but the ICO works.
It may be possible for mat2 to use icotool to extract images from .ico and .cur files and create a new file from them, but that would require some testing and studying the command line options as the format can include several images at different sizes plus some metadata, different if the file is an icon or a cursor.