For the Data-WorkAttach- attachments, the limit is 1 gig by default.
By default, file attachments are restricted to 1 gigabyte in size. You can set a larger or smaller limit with the prconfig.xml file setting: <env name="Initialization/MaximumFileUploadSizeMB" value="nn" /> (where nn is the size in megabytes)
Having said that, your system should have enough RAM and enough heap size to upload large files, and during the upload, performance can be severely degraded, can lead to OOM errors if the heap is exhausted.
After this change, try uploading larger files less than 1 GB and check the result. I couldn't intercept what is session out error. Provide a screenshot of this error or relevant log files. And also confirm if you are able to do other operations after hitting this error.
Connect-FTP can be used to transfer a file from local file path(Pega server) to remote file path(external server). That may not serve our purpose of attaching large files on work objects.