The developer documentation does not contain any restrictions, so you can assume that they are not:
File upload and download Safari on iOS supports file upload - this is the <input type="file"> elements - on iOS 6 and later.
Please note that in iOS 6-8, you are in any case limited to a photo library, so what works there.
IOS 9 has a new file upload dialog, which apparently allows you to download from various cloud storage services, which greatly expands the file download area in mobile Safari.
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