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Portal Walkthrough: Control 1.25 - MIME Type Restrictions for File Uploads

Last Updated: April 2026 Portals: Power Platform Admin Center (PPAC), Copilot Studio, Microsoft Defender for Cloud Apps, SharePoint Admin Center Estimated Time: 30-45 minutes (full Zone 3 walkthrough including per-agent and Defender for Cloud Apps configuration)

Prerequisites

  • Power Platform Admin role (canonical role per docs/reference/role-catalog.md) — required for PPAC environment settings
  • AI Administrator or environment-level Copilot Studio maker role — required for per-agent File Upload toggle and per-agent allowed file types
  • Entra Security Admin — required for Defender for Cloud Apps file policy configuration (Zone 3)
  • SharePoint Admin — required for tenant-level sync-client file blocking (defense-in-depth complement)
  • Documented governance-zone classification for each target environment
  • Documented per-agent file-type allowlist with business justification (for Zone 2/3 production agents)

Scope note: PPAC settings configured below apply at the environment level and govern attachments stored on the Dataverse Organization entity. Per-agent settings in Copilot Studio apply additional constraints within those environment bounds. SharePoint and Defender for Cloud Apps controls listed at the end are tenant-level complements, not substitutes.

Step-by-Step Configuration

Step 1: Navigate to Environment Settings (PPAC)

  1. Open Power Platform Admin Center
  2. Select Environments from the left navigation
  3. Select the target environment (repeat for each environment per zone)
  4. Click Settings in the top menu bar
  5. Expand Product → Privacy + Security

Portal path (April 2026): Power Platform Admin Center → Environments → [Environment] → Settings → Product → Privacy + Security. The Privacy + Security node now sits under Product; older guidance that lists it under Features is stale.

Step 2: Configure Blocked File Extensions

  1. Locate the Set blocked file extensions for attachments field
  2. Enter a semicolon-separated list of file extensions to block

  3. The following is a partial list of the most critical executable extensions. The complete Zone 1 baseline requires 44 extensions — use Set-FsiMimeConfig -ZoneTemplate zone1 from the PowerShell Setup for the full list:

    exe;bat;cmd;com;vbs;js;wsf;scr;pif;msi;dll;reg;inf;hta;cpl;msp;mst

    Partial List

    The inline list above covers only 17 of the 44 required Zone 1 extensions. Using this list alone will leave your environment under-protected. Apply the complete zone template via PowerShell or copy the full list from scripts/governance/mime-templates/zone1.json.

  4. Click Save to apply changes

Note: Zone 2+ environments should also block ps1. Zone 3 adds cab, gadget, ps1xml, ps2, ps2xml, psc1, psc2, isp, its, and rgs. The FsiMimeControl zone templates at scripts/governance/mime-templates/ contain the complete lists (44 extensions for Zone 1, 45 for Zone 2, 55 for Zone 3). For full compliance, use Set-FsiMimeConfig -ZoneTemplate zone1 from the PowerShell Setup playbook or copy the complete list from the zone template JSON file.

Step 3: Configure Blocked MIME Types (Zone 2+)

Zone 2 and Zone 3 only. Skip this step for Zone 1 environments.

  1. Locate the Set blocked mime types for attachments field
  2. Enter a semicolon-separated list of MIME types to block

  3. Recommended MIME types to block:

    application/x-msdownload;application/x-msdos-program;application/x-bat;application/x-cmd;application/x-vbs;application/javascript;application/x-powershell;application/x-msi

  4. Click Save to apply changes

Note: The list above covers the most common executable content types. The FsiMimeControl zone templates contain extended lists (15 types for Zone 2, 21 for Zone 3) including text/javascript, application/hta, application/msaccess, and others. For full compliance, use Set-FsiMimeConfig -ZoneTemplate zone2 from the PowerShell Setup playbook or copy the complete list from the zone template JSON file.

Step 4: Configure Allowed MIME Types (Zone 2+)

  1. Locate the Set allowed mime types for attachments field
  2. Enter a semicolon-separated allowlist of MIME types that are permitted

  3. Recommended allowlist for regulated environments:

    application/pdf;image/png;image/jpeg;image/gif;image/tiff;text/plain;text/csv;application/vnd.openxmlformats-officedocument.spreadsheetml.sheet;application/vnd.openxmlformats-officedocument.wordprocessingml.document;application/vnd.openxmlformats-officedocument.presentationml.presentation

  4. Click Save to apply changes

Important: When an allowlist is configured, only the listed MIME types are accepted. All other types are rejected regardless of the blocked list. Both Zone 2 and Zone 3 templates include image/tiff in the allowlist — see the zone template JSON files for the complete list.

Legacy Office Formats Not Included

The zone template allowlists include modern Office formats (.docx, .xlsx, .pptx) but not legacy binary formats (.doc, .xls, .ppt). If your organization exchanges legacy Office documents — common in FSI for regulatory correspondence and historical records — add application/msword, application/vnd.ms-excel, and application/vnd.ms-powerpoint to your environment's allowed MIME types list. Alternatively, use an exception request per the exception template.

Step 5: Review and Apply Zone Template

  1. Review the configuration against the governance level table below
  2. Verify the settings match the zone classification for the selected environment
  3. Document the applied configuration in your governance records
  4. Repeat Steps 1-4 for each environment within the zone

Step 6: Configure Per-Agent File Upload Settings (Copilot Studio)

Required for every file-upload-enabled agent in Zone 2 and Zone 3. PPAC settings establish the maximum allowable file types; per-agent settings apply additional least-privilege restrictions.

  1. Open Copilot Studio
  2. Select the target agent → Settings → Security
  3. Locate the File Upload toggle
    • Set to Off if the agent has no documented file-handling use case (recommended default for Zone 1 personal agents)
    • Set to On only when the agent's documented purpose requires file inputs
  4. If File Upload is On, configure Allowed file types to the minimum set required by the agent's documented purpose (e.g., .pdf only for a contract-summary agent — do not inherit the full environment allowlist by default)
  5. Capture screenshot evidence of the toggle state and allowed-type list for each production agent (store under maintainers-local/tenant-evidence/1.25/)

User-input limits to communicate to agent owners (Microsoft Learn, April 2026): PDFs uploaded by users at runtime are limited to <40 pages; TXT/CSV to <180 KB; images to 15 MB (4 MB on Direct Line). Files configured as knowledge sources by makers may be up to 512 MB, with 500 files per agent for local uploads and 1,000 files per agent for SharePoint/OneDrive sources (GA August 2025). Executable, audio, and video formats are not supported as knowledge sources and need not appear in the agent allowlist.

Step 7: Configure Defender for Cloud Apps File Policy (Zone 3 Magic-Byte Inspection)

Required for Zone 3. PPAC and per-agent settings inspect the declared file extension and MIME header. A file renamed invoice.pdf whose magic bytes are MZ (Windows executable) will pass PPAC checks. Defender for Cloud Apps (formerly MCAS) provides the true-content-type inspection layer.

  1. Open Microsoft Defender XDR portal → Cloud apps → Policies → Policy management → File policy
  2. Create policy → File policy
  3. Filter by App = SharePoint Online / OneDrive for Business (and any other connected app that hosts agent file inputs)
  4. Add filter: MIME type (true type) does not equal the approved Zone 3 allowlist (mirrors PPAC allowedmimetypes)
  5. Governance actions: Quarantine + Notify file owner + Notify SOC distribution list
  6. Alert: Create alert; set severity High; configure SIEM forwarding to Microsoft Sentinel
  7. Save policy and confirm it appears as Enabled in the policy list

Limitation: Defender for Cloud Apps file policies operate on connected SaaS apps via API connectors, with near-real-time (not synchronous) scanning. Files may be briefly accessible to agents before quarantine completes; pair with PPAC blocking to fail-fast at the environment edge.

Step 8: Configure SharePoint Tenant Blocked File Types (Defense-in-Depth)

  1. Open SharePoint Admin Center → Settings → Sync → Block upload of specific file types
  2. Add executable/script extensions matching your PPAC blocklist (e.g., exe, bat, cmd, vbs, js, ps1, dll, msi, scr, hta)
  3. Save

Important caveat: This SharePoint setting blocks the OneDrive sync client only. It does not block browser uploads to SharePoint, which means files reaching agent SharePoint knowledge sources via the browser are not filtered here. PPAC and Defender for Cloud Apps file policies remain the primary controls; this is a complementary layer for the sync-client vector.

Configuration by Governance Level

Setting Baseline (Zone 1) Recommended (Zone 2) Regulated (Zone 3)
Blocked File Extensions Yes — executable types Yes — executable types Yes — executable types
Blocked MIME Types Optional Yes Yes
Allowed MIME Types (Allowlist) Not required Required Required
DLP Policy for File Uploads Not required Yes Yes — with alerts
Sentinel Monitoring Not required Optional Required
Review Frequency Quarterly Monthly Weekly
Exception Process Informal Documented Documented with approval

Validation

After completing these steps, verify:

  • Blocked file extensions are configured for each environment (Microsoft defaults retained, plus organizational additions)
  • Blocked MIME types are configured for Zone 2 and Zone 3 environments
  • Allowed MIME types allowlist is configured for Zone 2 and Zone 3 environments
  • Per-agent File Upload toggle state is documented for every production agent (Zone 2 and Zone 3)
  • Per-agent Allowed file types are configured to the minimum set required by each agent's documented purpose (least-privilege)
  • Defender for Cloud Apps file policy with true-MIME (magic-byte) inspection is Enabled (Zone 3)
  • SharePoint sync-client blocked file types match the PPAC blocklist (defense-in-depth)
  • Configuration matches the governance level table for each environment zone
  • Screenshot evidence is captured under maintainers-local/tenant-evidence/1.25/ (gitignored)
  • Changes are documented in governance records and reviewed at zone cadence (Q/M/W)

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