Short answer. Only if the list meets all four requisites. Article 827 says a referenced document is not part of the will unless it existed when the will was executed, the will clearly describes and identifies it including its number of pages, it is identified by clear and satisfactory proof, and it is signed on every page.
What the law says
such document or paper shall not be considered a part of the will unless the following requisites are present
Civil Code, Article 827 — Incorporation by Reference. Read the full provision →
What the law says
The document or paper referred to in the will must be in existence at the time of the execution of the will
Civil Code, Article 827 — Incorporation by Reference. Read the full provision →
The default is exclusion
Read the opening of the article carefully, because it is written the wrong way round from how families read it. A referenced paper shall not be considered a part of the will unless the following requisites are present. So mentioning a list in the will does not pull the list in; it merely opens the possibility. The four conditions that follow are cumulative, and a list that misses any one of them stays outside the will however clearly it records what the testator wanted. That is a hard result, and it is deliberate: an outside paper is far easier to substitute or amend than a will.
It must already have existed, and be pinned down
The first requisite is that the paper must be in existence at the time of the execution of the will. A list drawn up afterwards can never qualify, which disposes of most of these documents immediately, since the usual pattern is a will made first and a list of chattels written later as the testator's plans firmed up. The second requisite is that the will must clearly describe and identify the paper, stating among other things the number of its pages — a detail designed to make substitution obvious, and one that a will referring vaguely to "my list" does not meet.
Proof of identity, and a signature on every page
The third requisite is that the paper be identified by clear and satisfactory proof as the document referred to in the will. The fourth is the one that defeats almost every handwritten list: it must be signed by the testator and the witnesses on each and every page, with an exception only for voluminous books of account or inventories. A private list of who gets which piece of jewellery, written by the testator alone and unsigned by the witnesses to the will, does not satisfy that condition no matter how plainly it expresses her intentions.
If the list fails, the items do not vanish
A list that falls outside the will simply has no dispositive effect; the property it dealt with is distributed under the rest of the will, or, if the will did not otherwise dispose of it, under the rules of intestate succession. That is worth knowing before a family starts arguing over the list itself. Keep the original list with the will and the envelope it was found in, note whether it is dated and who signed it, and compare the will's own description of it against the paper actually in your hands, page count included.