Work Context: How to Attach Files and Ground Cowork
Post 12 of 14
Introduction
Cowork’s output quality is largely a function of the context you provide. A vague prompt against an empty workspace produces generic text; the same prompt with three well-chosen attachments produces something you can actually send. This post covers how to attach files, what Cowork can read, and a few practical compounding patterns.
Attaching Files the Right Way
Three ways to add context.
- Drag and drop from your device directly into the chat input.
- Add work context to pull from OneDrive, SharePoint, or Teams via a picker. This covers files, people, meetings, and chats.
- Reference by path in the prompt text itself (“use
/Documents/Brand/guide.pdf”).
Supported file types (per Microsoft Learn). Cowork can read:
- Word:
.docx,.doc,.docm,.dot,.dotx,.odt,.rtf - Excel / tabular:
.xlsx,.xlsm,.xls,.csv,.ods - PowerPoint:
.pptx,.pptm,.ppt,.odp - Markdown and plain text
- Images (including for visual reference)
- Code files and config files
- Jupyter notebooks (
.ipynb) - Audio and video (for the relevant skills)
- Archives (zipped bundles)
Size and location rules.
- Each attachment must be under 200 MB.
- Files must live in OneDrive, SharePoint, or Teams — Cowork cannot reach local files on your device, only those you upload.
- Cowork cannot read encrypted files, even when you have access through your own keys.
- Cowork cannot delete files in OneDrive or SharePoint.
What good grounding looks like. Three attachment patterns that change the output quality noticeably:
1. Brand and tone kit. A folder containing your style guide, a sample memo, and a glossary. Attach it to any writing task and tell Cowork: “Match the tone and structure of the attached memo.” Your output reads like your team’s, not like generic AI.
2. Context pack for a recurring task. For a weekly report or monthly review, assemble a folder with the last period’s deliverables, a definitions file, and the data source. Attach the whole folder. Cowork produces consistent output across cycles.
3. Source-of-truth pinning. For a fact-heavy deliverable — compliance memo, customer briefing — attach the authoritative document and tell Cowork: “Only state facts from the attached document. Flag anything I need to verify separately.” This constrains hallucination risk significantly.
How Cowork uses attachments. When you attach a file, Cowork reads it as grounding for the current task. It does not automatically re-read old attachments in later conversations — each task is fresh. To reuse context, reattach or reference the OneDrive path.
Work IQ grounding (automatic). Separately from explicit attachments, Cowork uses Microsoft’s Work IQ layer to understand your patterns, relationships, and content across the Microsoft 365 graph (mail, calendar, Teams, files you own or have recent access to). This is automatic and permission-bound.
Prompt-attachment interplay. A prompt can address specific attachments:
- “Use the first attached PDF as the source of truth; use the second PDF only for formatting reference.”
- “Ignore attachment 3 for this task — it’s background only.”
- “Summarise all attachments into a single brief.”
Verification pattern. For anything externally-facing, ask Cowork to include citations back to the source items. Outputs from Deep Research and Enterprise Search do this by default; for other skills, request it explicitly.
Troubleshooting attachments.
- File above 200 MB → split it or convert it.
- File encrypted → Cowork will skip it and say so; decrypt or use an unencrypted copy.
- File on a local drive → upload to OneDrive first.
- File in a SharePoint site the user has no access to → Cowork respects existing permissions and will not surface it.
Takeaway
The gap between average and excellent Cowork output is usually one or two good attachments away. Build reusable context packs for recurring work. Pin sources of truth. Cite when it matters.
Start the conversation