I imported a large files (about 214 MB).
After a while processing, I got a message reading something like "Import failed because the maximum script time has been reached. If you are importing a big file, submit the same file again and import will resume". (I'm citing by memory).
I did as instructed: imported the very same file again, but import immediately failed with the message "cannot create database xxxxx: database already exists" (this is a database that did not exist previous to the import and that was contained in the file being imported).
So, either resume is supposed to be supported, in which case it's broken, or it is not, in which case the error message when the script execution time shouldn't lie promising you can reupload the same file and have the import resumed.