I had this same problem with the McGIll application! I just provided an abstract of my statement of purpose when it asked for the "brief research summary," and subsequently freaked out when I realized I couldn't change it/had no idea what they intended for me to upload.
I e-mailed the graduate coordinator asking her about it, but haven't heard back.
When looking again at the app requirements on their website, the wording makes it seem like you're supposed to submit hard copies of the statement of purpose in the mail, with the rest of the required materials like the writing samples, etc.
It seems ridiculous that they wouldn't accept a statement of purpose that was submitted with the rest of the required materials.
At least, that's what I'm hoping.