Added a new feature: maximum total upload file size (default: 10M).

The default might have to be raised or the feature might have to be turned of by default to make it more backwards compatible.

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Form now keeps track of the total size of the uploads to keep it under
a set limit (default to 10M) e.g. to keep the total size of the message
within some email server limits.

Thus might be backwards incompatible unless the total size is redefined.