You can probably not upload files larger than about 300.00 MiB in size.
Alternatively, you can use a file you already uploaded (by SFTP or SCP) to the project's incoming directory (/var/lib/fusionforge/chroot/home/groups/darn/incoming). This direct sftp:// link only works with some browsers, such as Konqueror. Choose an already uploaded file:
No uploaded file available.
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