You can set ThumbnailSize on the discussion container module, although this was a deep down property but we have exposed that too. It will be available in today's build.
1) at forum post attachments thumbnails (both at read and at post edit view) are a bit too small. We've stretched them in the layout, but wonder where I set the default thumbnail size for when the thumb is generated so that they don't show blurry with the extra stretch
We have added mp4 file extension to be supported by default, thanks. As for the file type icons you need to change or add them at the following location: "/App_Themes/Common/img/FileTypeIcons
", at the moment you will not be able to easily add new types to the list but we have extended this and it will be available in the today's nightly build.
you can't upload .mp4 files (not in list of allowed extensions) to forum posts, but you can for example .wmv and it does show a video icon for the attachment (wonder how one can relate such icons to file extensions and possibly add more filetypes with their own icons [e.g. in our case I want to associate a .clipflair.zip - note that it should be clever enough to not see this as a plain .zip file to an icon])
We need to check ... but from top of my mind No.
3) not sure if I can add custom actions like "Play" to open the attached file via some URL e.g. http://myplayer?activity=someURL - ideally I would be able to set the default action for a file type and have popup menu for when clicking an attached file to select Download or other actions (e.g. say "Archive" to keep at my own online archive etc. [btw, do users have a filespace of their own on monox?])
4) when a file with spaces in its name is attached to a forum, then when mouse moves over its icon it shows the correct name, but when clicked to download it (tried with latest Mozilla on Win7) you get only the first part of the name (till the first space char) without a file extension (so the user may think the file is broken)