I just finnished uploading a large amount - coupple thousands - of JPG images to an FTP saved from RAW and discovered that PM (as it seems) first copies all of the original RAW files (taking up a large amount of disk space duplicating each image) and only then saves from the copied images the JPGs it will send.
I think it is a waste of time and can be a great sitback in workflow since I had to do 3 or 4 times the first part (copying) after recognizing the problem I did not have disk space enough for duplicating all RAW.
My second problem came when I tried to upload them in a differend resolution (I have to send thumbnail and display images to a web gallery generated by the server from the uploaded images). Since PM doesn't delet its temporary files nor after its internal conversion of images nor after finnishing the upload. So I had to quit PM to have it free up the used disk space then open up the same contact sheet, set the same filtering, select the same images, and start the (not entirely sam) FTP upload with PM repeting the same copy using up the same disk space.
As I see PM uses the copied originals only to convert them in the background using the upload settings. After it is done with that the originals aren't needed since the FTP upload cannot be resumed, restarted, or modified in any way. Only stopping it is possible that doesn't need the originals, and once stopped restarting it won't use the copy done before but start it from scratch.
I think this behaviour should be revised somehow with either PM saving the images to send instead of the originals (this would take up less disk space), and/or should it free up the space after not needing the files any more (either after it's done converting for send, or after finnishing the send), or at least have an option to manually free up the space taken after the upload is complete.
I hope this can be solved somehow.