* Refactor widget items to use same interface
* Support audio, video, generic file for pasting
* Add utils function for human readable file sizes
* Set correct MIME type for media messages
This change also determines the size of the upload once from the
ContentLengthHeader, rather than seeking the QIODevice and asking for
its size. This prevents any future trouble in case the QIODevice is
sequential (cannot be seeked). The MIME type is also determined at
upload once, rather than using the QIODevice and the underlying data
inside.
* Allow for file urls to be used as fall-back
This fixes an issue on macOS which uses `text/uri-list` for copying
files to the clipboard.
fixes#228
