Skip to content

MIME Type Restrictions Solution - Customer Delivery Checklist

Files to Include in Customer Package

1. Documentation

  • docs/flow-configuration.md — Main technical documentation (Executive Summary + Technical Details)

2. Solution Components (Source Files)

Files located in src/, docs/, scripts/, and templates/ directories:

Server-Side Validation: - [ ] src/ValidateMimeTypePlugin.cs — Dataverse pre-validation plugin (C# source) - [ ] docs/build-instructions.md — Step-by-step guide to build the plugin DLL from source - [ ] docs/build-and-sign.md — dotnet build, strong-name signing, and verification guide - [ ] src/ValidateMimeTypePlugin.csproj — SDK-style .NET Framework 4.6.2 project file - [ ] templates/mime-config.json — MIME type allowlist/blocklist configuration

Data Policy Reference: - [ ] templates/dlp-policy-template.json — Connector-classification reference for Power Platform data policies

Sentinel Monitoring: - [ ] scripts/query-mime-blocks.kql — KQL query for blocked upload events (30-day summary) - [ ] templates/high-volume-blocks.json — Sentinel alert rule for high-volume block patterns - [ ] scripts/query-exception-usage.kql — KQL query for exception usage tracking

3. Packaging Instructions

Option A: Create ZIP Archive 

# From the mime-type-restrictions directory:
zip -r MIME-Type-Restrictions-v1.2.1.zip \
  docs/flow-configuration.md \
  docs/build-instructions.md \
  docs/build-and-sign.md \
  src/ValidateMimeTypePlugin.cs \
  src/ValidateMimeTypePlugin.csproj \
  templates/mime-config.json \
  templates/dlp-policy-template.json \
  scripts/query-mime-blocks.kql \
  templates/high-volume-blocks.json \
  scripts/query-exception-usage.kql

Option B: Create Structured Folder 

MIME-Type-Restrictions-v1.2.1/
├── docs/
│   ├── flow-configuration.md
│   ├── build-instructions.md
│   └── build-and-sign.md
├── src/
│   ├── ValidateMimeTypePlugin.cs
│   └── ValidateMimeTypePlugin.csproj
├── templates/
│   ├── mime-config.json
│   ├── dlp-policy-template.json
│   └── high-volume-blocks.json
└── scripts/
    ├── query-mime-blocks.kql
    └── query-exception-usage.kql

4. Email Template

Subject: MIME Type Restrictions for File Uploads - Solution Delivery v1.2.1

Body:

Hi [Customer Name],

Please find attached the MIME Type Restrictions for File Uploads solution package, version 1.2.1.

This solution provides defense-in-depth validation of file uploads in Copilot Studio agents
through server-side magic byte inspection, Power Platform data policy guardrails, and Sentinel monitoring.

Package Contents:
- docs/flow-configuration.md — Complete technical documentation with:
  • Executive Summary (problem statement, solution overview, business value)
  • Technical Details (architecture, 3-layer enforcement, components)
  • Configuration and Prerequisites (plugin registration, data policy setup)
  • Deployment validation steps
  • Operational guidance and troubleshooting
  • Magic byte reference and regulatory alignment

- Solution component files:
  • Dataverse pre-validation plugin (C# source + project file)
  • MIME type configuration (JSON allowlist/blocklist)
  • Data policy reference (Power Platform connector classifications)
  • 2 Sentinel KQL queries and 1 scheduled analytics rule template (monitoring, alerting, exception tracking)

Key Capabilities:
✓ Server-side magic byte inspection (defense against disguised executables)
✓ Blocked signature detection (PE, ELF, Mach-O, Java class files)
✓ OpenXML deep inspection for Office documents (DOCX, XLSX, PPTX)
✓ Power Platform data policy connector guardrails
✓ Sentinel monitoring for blocked upload attempts
✓ High-volume attack detection (>10 attempts per user per hour)

Architecture:
• Layer 1: Power Platform data policy (connector classifications)
• Layer 2: Dataverse Plugin (pre-validation with magic byte inspection)
• Layer 3: Sentinel Monitoring (blocked event aggregation and alerting)

Business Value:
• Significantly reduces malware distribution risk through multi-layered validation
• Helps reduce data exfiltration risk from file-based steganography
• Enables security operations teams to detect upload abuse patterns
• Supports regulatory examinations with MIME restriction evidence

Regulatory Support:
• NIST 800-53 SI-3 — Malicious Code Protection
• FINRA 3110 — Supervision
• SEC 17a-4 — Recordkeeping

Next Steps:
1. Review the docs/flow-configuration.md file (Section 2: Technical Details)
2. Build Dataverse plugin DLL from C# source (Visual Studio required)
3. Register plugin using Plugin Registration Tool:
   - Entity: annotation (Note)
   - Message: Create
   - Stage: Pre-Validation (10)
   - Configuration: Paste MimeConfig.json content
4. Configure data policy connector classifications in the Power Platform admin center or with PowerShell
5. Deploy Sentinel queries and alert rule to Log Analytics workspace
6. Test with allowed and blocked file types (PDF vs EXE)
7. Schedule deployment planning session (recommended: 2-3 hours)

CRITICAL CONFIGURATION REQUIREMENTS:

1. **Plugin Configuration:**
   - MimeConfig.json must be pasted into plugin step configuration (unsecure or secure)
   - Enforcement mode: "Block" for production, "TestWithNotifications" for testing
   - Max file size: 10 MB default (adjust based on business requirements)

2. **Allowed MIME Types:**
   - Default allowlist: PDF, PNG, JPEG, GIF, WebP, TXT, CSV, DOCX, XLSX, PPTX
   - Customize based on organization's legitimate file upload needs
   - Require business justification for new MIME type additions

3. **Blocked Signatures:**
   - Default blocklist: PE/DOS (EXE), ELF, Mach-O, Java class files
   - DO NOT remove blocked signatures without security team approval
   - Add custom signatures if organization-specific threats identified

4. **Data Policy Connector Scope:**
   - Pilot connector classification changes in a non-production or scoped environment group
   - Review maker and runtime impact for 2-4 weeks
   - Broaden the policy only after validating required Copilot Studio and upload paths

5. **Sentinel Workspace:**
   - Requires Dataverse audit logs flowing to Log Analytics workspace
   - Diagnostic settings: "Send to Log Analytics" enabled
   - 15-minute ingestion delay expected (not real-time)

Please let me know if you have any questions or need clarification on any aspect
of the solution.

Best regards,
[Your Name]

5. Pre-Delivery Validation

Before sending to customer, verify:

  • All 10 files are included (flow guide, build guide, build/sign guide, C# source, project file, MIME config, data policy reference, 2 KQL queries, 1 Sentinel alert rule)
  • docs/flow-configuration.md renders correctly in Markdown viewer
  • C# source compiles without errors (test build in Visual Studio)
  • MimeConfig.json is valid JSON (use JSON validator)
  • No sensitive data in files (tenant IDs, user emails should be placeholders)
  • Version numbers are consistent (v1.2.1) across all files

6. Files NOT to Include

Do NOT include these repository management files: - ❌ README.md (internal reference only) - ❌ CHANGELOG.md (version history for our tracking) - ❌ .git/ folder (version control)

7. Customer Requirements Reminder

Remind customer they will need:

Licensing: - Microsoft 365 E5 or E5 Compliance (for DLP policies and Sentinel) - Power Platform Admin permissions - Power Apps Premium (for Dataverse) - Microsoft Sentinel workspace (for KQL queries)

Tools: - Visual Studio 2019+ (to build plugin DLL from C# source) - Plugin Registration Tool (to register Dataverse plugin) - PowerShell 7.2+ (to review or update data policy connector classifications) - Azure portal access (to configure Sentinel queries)

Permissions: - Dataverse System Administrator (per-environment for plugin registration) - Power Platform Admin (tenant-wide for data policy management) - Security Reader (Log Analytics workspace for query execution) - Security Administrator (Sentinel workspace for alert rule creation)

Prerequisites: - Dataverse audit logging enabled in all environments - Diagnostic settings: "Send to Log Analytics" enabled - Log Analytics workspace connected to Sentinel

Optional Component: - FsiMimeControl PowerShell module — Available in FSI-AgentGov repository under scripts/governance/. Provides bulk configuration management, deployment validation, and zone template support. Not required for core functionality but recommended for multi-environment deployments.

8. Follow-Up Support

Offer these follow-up services: - Plugin build and registration assistance (2-4 hours recommended) - MimeConfig.json customization workshop (1 hour) - Data policy testing and validation (2 hours) - Sentinel dashboard creation for upload monitoring - Quarterly review of blocked upload trends and policy tuning

9. Important Customer Guidance

Critical Configuration Steps:

  1. Build Plugin DLL:
  2. Customer MUST build the plugin from C# source code
  3. Visual Studio project type: Class Library (.NET Framework 4.6.2)
  4. Required NuGet packages: Microsoft.CrmSdk.CoreAssemblies (9.0.2+), System.Text.Json (8.0.0+ — must be ILMerged for sandbox deployment)
  5. Required assembly reference: System.IO.Compression

  6. Output: FsiAgentGovernance.Plugins.dll

  7. Plugin Registration:

  8. Use Plugin Registration Tool (download from Microsoft NuGet)
  9. Register on annotation entity, Create and Update messages, Pre-Validation stage (10)
  10. REQUIRED: Register a Pre Image named PreImage on the Update step with attributes mimetype, filename — without this, partial updates that omit those columns will be fail-secure-blocked by the plugin
  11. CRITICAL: Paste entire MimeConfig.json content into plugin step configuration

  12. Without configuration, plugin will throw error on first upload

  13. MimeConfig.json Customization:

  14. Review default allowlist (10 MIME types) and adjust based on business needs
  15. DO NOT remove blocked signatures without security team approval
  16. Add magic byte patterns for new allowed types (consult file format specifications)

  17. Test magic byte validation with sample files before deploying

  18. Data Policy Testing:

  19. Pilot connector classification changes in a non-production or scoped environment group
  20. Review maker and runtime impact for 2-4 weeks
  21. Document exceptions for business-justified connector paths

  22. Broaden policy scope only after validation is complete

  23. Sentinel Alert Tuning:

  24. Default threshold: 10 blocked uploads per user per hour
  25. Adjust based on organization size and upload volume
  26. Too low threshold = alert fatigue, too high = missed attacks
  27. Review incident history quarterly and adjust threshold

10. Deployment Validation Checklist for Customer

Provide this checklist to customer for post-deployment validation:

Plugin Build and Registration:
□ Visual Studio project created (Class Library .NET Framework 4.6.2)
□ NuGet packages installed: Microsoft.CrmSdk.CoreAssemblies, System.Text.Json
□ Assembly reference added: System.IO.Compression
□ ValidateMimeTypePlugin.cs compiled successfully
□ DLL output generated: FsiAgentGovernance.Plugins.dll

Plugin Registration:
□ Plugin Registration Tool connected to Dataverse environment
□ Assembly registered (Isolation: Sandbox, Location: Database)
□ Plugin steps registered on annotation.Create and annotation.Update, Pre-Validation (10)
□ Pre Image named `PreImage` registered on the Update step with `mimetype, filename`
□ MimeConfig.json pasted into plugin step configuration (unsecure or secure)
□ Plugin step status: Active

MimeConfig.json Validation:
□ JSON syntax validated (no parsing errors)
□ Enforcement mode set: "Block" for production
□ Max file size configured: 10485760 bytes (10 MB) or custom
□ Allowed types reviewed and customized for organization
□ Blocked signatures NOT removed (security risk)

Data Policy Deployment:
□ dlp-policy-template.json reviewed as a connector-classification reference
□ Data policy updated in the admin center or with Microsoft.PowerApps.Administration.PowerShell
□ Risky upload-capable connectors classified as Blocked or Non-business as required
□ Policy scope configured: target environments or environment groups
□ Required Copilot Studio connectors remain in compatible data groups

Sentinel Queries:
□ Log Analytics workspace connected to Dataverse audit logs
□ query-mime-blocks.kql saved as function: MimeTypeBlocks
□ query-exception-usage.kql saved as function: MimeExceptionUsage
□ high-volume-blocks.json imported as alert rule
□ Alert rule enabled and configured (1-hour frequency, Medium severity)

Test Execution:
□ Test 1: Upload allowed PDF → Success (annotation created)
□ Test 2: Upload blocked EXE → Blocked (error message displayed)
□ Test 3: Upload ZIP (not in allowlist) → Blocked
□ Test 4: Upload disguised EXE (renamed to PDF) → Blocked (magic byte mismatch)
□ Test 5: Data policy blocks or separates a risky upload connector path → policy violation observed
□ Test 6: Sentinel query returns blocked events → Results displayed

Plugin Trace Log Review:
□ Trace logs show successful validation for allowed files
□ Trace logs show blocked signature detection for EXE
□ Trace logs show magic byte consistency validation
□ No errors or exceptions in trace logs during normal operation

Operational Readiness:
□ Security operations team trained on Sentinel dashboard
□ High-volume alert response procedure documented
□ Exception request process documented (business justification required)
□ Quarterly policy review scheduled (MIME type allowlist tuning)

Package Version: v1.2.1 Release Date: Unreleased (2026-Q2) Solution: MIME Type Restrictions for File Uploads