In the sentence:
"Please find attached the document containing the list of items."
Is the usage of "the" after "attached" forbidden by some rule of the English language?
Your sentence is perfectly fine.
The following is also correct:
Please find the attached
thedocument containing the list of items.
There is not such rule, as you mention it. But there are rules about when and how to use the definite article ("the").
"Please find attached" is basically a set phrase now in English. It can be analysed for parts of speech, certainly, but it is not thought of in such terms by native speakers. I'm not even sure of its origins, but you might think of it as a contraction of:
Please find, attached to this email (or letter), ...
Thus any appropriate noun phrase can follow it - with an article or not.