Отправитель забыл предупредить, что во вложении ещё и фотографии, поэтому получатель сначала ничего не понял.

Russian often leaves out an object if it is clear from context. Here, the implied object is получателя — the recipient. So the full idea is:

Отправитель забыл предупредить (получателя)...

In English, we often need to say warn him / tell the recipient, but Russian can omit it more easily when it is obvious.

Literally, предупредить often means to warn or to notify in advance. In this sentence, it is closer to:

• to let someone know in advance
• to mention beforehand
• to inform

So here it does not sound dramatic, like warning about danger. It simply means the sender failed to tell the recipient ahead of time that there were photos attached too.

These are similar, but not identical.

• не предупредил = didn't warn / didn't mention
• забыл предупредить = forgot to warn / forgot to mention

The sentence uses забыл предупредить to show that it was an oversight, not just a bare fact that he failed to do it.

Because the speaker is talking about one complete act: informing the recipient.
After забыл, Russian often uses the aspect that matches the intended action.

• предупредить (perfective) = to give the warning/information as a complete act
• предупреждать (imperfective) = to be warning, to warn repeatedly, or in a more process-like sense

Here, the sender forgot to perform that single completed action, so предупредить is the natural choice.

Во is just a pronunciation variant of в. Russian often uses во before certain consonant clusters because it is easier to say.

So:

• в and во mean the same thing here
• во вложении is the normal, idiomatic phrase

This is especially common in formal written contexts like emails.

It is prepositional case.

• dictionary form: вложение
• after в / во when it means in and answers where?, it becomes вложении

So:

• во вложении = in the attachment

This is a fixed and very common expression in business or email Russian.

Ещё и means something like:

• also
• as well
• on top of that
• sometimes even and even

Here it suggests that the attachment did not contain just one thing; it contained photographs too.

So the nuance is: there were photographs in the attachment as well, not just whatever the recipient was expecting.

Because it functions as the subject of an implied verb such as есть or были.

The structure is basically:

...что во вложении [есть / были] ещё и фотографии.

So фотографии is nominative plural because it is the thing that is/was in the attachment.

The first comma is there because что introduces a subordinate clause:

• Отправитель забыл предупредить, что...

The second comma is there because поэтому connects the result clause:

• ..., поэтому получатель...

So the sentence is divided into logical parts:

1. the sender forgot to mention something
2. what exactly he forgot to mention
3. the consequence of that

This is normal Russian grammar. Russian uses negative concord, which means multiple negative elements appear together in one sentence.

So:

• не понял = did not understand
• ничего = nothing / anything in a negative sentence

Literally it looks like didn't understand nothing, but that is not how it works in Russian. The actual meaning is:

didn't understand anything

This is completely standard.

Because понял is perfective and focuses on the result: the recipient failed to grasp what was going on.

• не понял = didn't understand / failed to understand
• не понимал = was not understanding / didn't understand for some period of time

With сначала (at first), не понял works very well because it suggests an initial failed moment of understanding.

Yes, they are fairly neutral-to-formal and are very common in contexts like:

• letters
• parcels
• email
• delivery
• documents

They correspond to:

• отправитель = sender
• получатель = recipient

In casual speech, people might use less official wording depending on context, but these two nouns are completely natural here.