[Swift] Abort uploads bigger than x MiB

[edda]

The swift upload dialog has a note that only files smaller than 1 MiB are accepted and to use a different client for larger files. However it seems that this size limit is not enforced at the moment.

Fix: Abort uploads when more than 1 MiB is received. Potentially this limit can be increased a bit to max. 10 MiB. If we decide to do this, also update the note above.