Hi,
flexibleVM2Global.css is an ordinary CSS file, i have no clue why why you got that error message. Are you getting that error only while uploading flexibleVM2Global.css OR for the entire files ? CSS are like a file with TXT format, they are old-fashioned text based files (not rich-text).
I am also using MAC, and i am also using "TRANSMIT v4.2" FTP client, I have never got an error like that before. You can try to use FileZilla which is freeware FTP client, and don't make any change in the configuration, just install the FileZilla, type your FTP server in the "quick connect" dialog box and try to upload, see if you get the same error message.
I am pretty sure, this error has something to do with the "TRANSMIT" configuration (it might have to do with wrong ASCII or Binary file extensions selection for CSS in the Configuration, can't be sure). -OR- you are trying to upload the files to the wrong directory (the directory that you have no access to make any changes)
Regards.