Don't just silently fail to upload an image

[pheara]

If an attachment is too big, there should be feedback to the user and suggestions on how to make it smaller (e.g. "use our phones photo-edit tools usually available in the camera app", "use paint", etc)

[pheara]

#2662 will make this obsolete as far as resizing is regarded. If the upload fails for another/any reason, there should still be a message.

[quasarchimaere]

although this happens mostly on file or image upload the underlying problem seems to be the websocket message size limitation (afaik)

[quasarchimaere]

scaling would only work for images but not for general file upload. we need to figure out a way to handle bigger details/ long content with a different approach, sending should use a http-post instead of a ws send, and receiving a big ws message should just send a sort of link that can be retrieved via an http-get as well.