A file can be attached to a conversation and asked about directly, rather than pasting its contents in.
What can be attached
Documents as PDF, Word, Excel, CSV, plain text or Markdown.
Images as JPEG, PNG, WebP, GIF, HEIC or SVG — including a photograph taken on a phone.
Images and documents are handled differently
Images are read visually. The assistant looks at the picture, so a photograph of a form, a whiteboard, a receipt or a piece of equipment can be asked about immediately.
Documents are read as text, broken into passages so the relevant part of a long file can be found rather than only its beginning. That is why a large document can be asked specific questions rather than summarized whole.
Scanned documents
A scanned PDF is a picture of text rather than text, so it is examined visually before it can be searched. That takes longer than a digital file, and the result depends on the scan.
A flat, evenly lit image of the whole page reads far better than one taken at an angle or in low light. Re-taking a poor photograph is usually quicker than correcting what came out of it.
A statement or a receipt also reaches your books
Most attachments are read and answered about, and that is the end of it. Financial documents are the exception, because leaving them in a conversation would mean the figures were discussed but never recorded.
A bank or card statement is recognized by its own content rather than its filename, read for its transactions, and prepared as an import against the account it belongs to. Nothing is written to your books at that point. The transactions wait under Accounting → Reconciliation, where the count, the date range, the totals and anything that looks like a row you already have are shown before you decide — the same preview as uploading a bank statement.
A receipt or an invoice is held for confirmation against the payment it belongs to, rather than posted.
Both stop short of changing a figure on their own. A document that arrives in a conversation has no approval attached to it, and financial records should not move because a file was attached.
What it is useful for
Asking a supplier agreement what the notice period is. Checking a quote against the specification. Getting numbers out of a spreadsheet somebody sent.
The alternative is a document you already keep in Drive, which does not need attaching each time — see Google Drive.
Size
Large files take proportionally longer to become available. Very large ones may need to be split.
If nothing seems to have arrived
Usually format or legibility rather than the upload itself — see an upload did not appear.