[GSoC idea] Chat file picker

[nickvergessen]

Next to the chat input field there should be something like an attachment icon. After clicking it, the file picker should open. When a file or folder is selected the file should be publicly shared and the link should be automatically posted into the chat, maybe even including a preview and some infos about the file/folder (size, type, etc.)

@jancborchardt want to prepare some mock ups?

[nickvergessen]

Implemented with #1151