Hi:
I'm totally confused.
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File attachement controls ARE allowed in browser forms
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If your data structure is correct, there is no reason the previously attached attachment would repeat
Simply drag a repeating section onto the form, then drag a file attachment control into that repeating section. InfoPath will generate a data source with the base64 field INSIDE the repeating group. If you want to create the data source yourself, first, create a repeating group and then, inside it, add the base64 field.
Mind you, if you already have a form in use with the field as a non repeating field and not in a repeating group, moving it will probably cause data loss in existing forms.