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Safari on the iPad can not upload to a site. This is because the iPad does not have a general file storage area for uploads (or downloads); so Safari grays out the Choose File button.
iCab Mobile (a third party web browser) gets around this by creating it's own file space. You have to copy the file to iCab first (using Open In or another method), then you can visit the site and upload the file. iCab Mobile works with most sites, but not all. The only way to find out it to try.