Quick fix: Some file thumbnails (especially HEIC, RAW, and some PDFs) appear black because Windows lacks the necessary preview handler. Install HEIF Image Extensions + Raw Image Extension from Microsoft Store. For PDFs, install Adobe Reader or set Edge as the default PDF handler. For other corrupted thumbnails, delete %LocalAppData%\Microsoft\Windows\Explorer\thumbcache_*.db and restart Explorer.

You open a folder of photos and most show proper thumbnails, but some are black squares or generic file icons. Or a PDF folder where some files have preview images and others don’t. The cause is one of three: missing codec/preview handler for that file type, corrupted thumbnail cache, or per-file thumbnail generation failure.

Symptom: File thumbnails show as black squares, generic icons, or fail to generate; specific file types are affected.
Affects: Windows 11 (and Windows 10) File Explorer thumbnail rendering.
Fix time: ~10 minutes.

What causes this

File Explorer thumbnails are generated by registered preview handlers. Each file type maps to one or more handler DLLs that read the file format and produce a preview bitmap. Windows ships with handlers for common formats (JPG, PNG, MP4, GIF) but lacks handlers for newer or third-party formats: HEIC (iPhone photos), HEIF, AVIF, RAW (camera raw), WebP, OneDrive cloud-only placeholders, and PSD.

Additionally, Windows caches generated thumbnails in thumbcache_*.db files. These can become corrupted, causing thumbnails to render as black even when the format is supported. Cleaning the cache forces regeneration.

Method 1: Install missing codec extensions from Microsoft Store

The fix for format-specific thumbnail failures.

Open Microsoft Store.
Search for and install these extensions (all free):
- HEIF Image Extensions (Microsoft) — for iPhone .heic photos
- HEVC Video Extensions — for .heic content with HEVC video; ~$0.99 from Microsoft, or free via OEM bundle
- Raw Image Extension (Microsoft) — for camera RAW files (.cr2, .nef, .arw, .raw)
- AV1 Video Extension — for AV1-encoded videos
- WebP Image Extensions — for WebP images
After installing each: open File Explorer to a folder with those file types. Wait 10–30 seconds for thumbnail regeneration.
For PSD files: install PSD Codec from third-party developer (Ardfry, paid) or use a thumbnail tool like SageThumbs (free).
For PDF thumbnails: ensure Adobe Reader is installed and set as default PDF handler. Edge can show PDF previews but Adobe Reader’s shell extension generates inline thumbnails more reliably.

Format-specific issues clear up once the right extension is installed.

Method 2: Rebuild the thumbnail cache

For when supported formats still show black thumbnails.

Open Disk Cleanup: Win + R → cleanmgr.
Pick the C: drive. Click OK.
Tick Thumbnails (in the list). Click OK → Delete Files.

Alternative manual method — open Terminal:

taskkill /f /im explorer.exe
del /f /s /q /a %LocalAppData%\Microsoft\Windows\Explorer\thumbcache_*.db
del /f /s /q /a %LocalAppData%\Microsoft\Windows\Explorer\iconcache_*.db
start explorer.exe

Wait 30 seconds for Explorer to re-launch. Open a folder — thumbnails regenerate progressively.
For specific folders that still misbehave: right-click an affected file → Properties → General tab → Advanced → uncheck Compress contents to save disk space. NTFS compression occasionally breaks thumbnail generation.
Disable then re-enable thumbnail generation in Folder Options: open Folder Options (search Start menu) → View tab → tick Always show icons, never thumbnails → Apply → OK. Then untick the same option → Apply → OK. This forces Windows to re-evaluate thumbnail generation logic.

Cache rebuild is the right fix for corrupted-cache cases.

Method 3: Per-file thumbnail issues — check the file itself

For when only specific files show black thumbnails while others of the same format work.

Open the file in its expected app (HEIC in Photos, RAW in Photo Viewer, MP4 in Movies & TV). If the file opens fine, the file is OK but thumbnail generation specifically is failing.
Check file size: extremely small (corrupted) or extremely large files sometimes fail thumbnail generation due to resource limits. Try opening and re-saving the file.
Check for read-only or system attribute: attrib <filename> in Terminal. Files with the System attribute don’t get thumbnails. Remove with attrib -s <filename>.
For OneDrive cloud-only files (cloud icon overlay): thumbnails are downloaded from OneDrive’s cloud-rendered preview, which can take longer. Mark Always keep on this device to force local download — thumbnails generate from the local copy.
For video files: install codec packs. K-Lite Codec Pack (free) includes thumbnail generators for less-common codecs (MKV containers, VP9, FLV).
For Office files (DOCX, XLSX, PPTX): Office must be installed for inline thumbnails. Without Office, you get generic file icons. The free Office mobile apps don’t generate desktop thumbnails — install Microsoft Office or LibreOffice (which also generates thumbnails).

Per-file fixes for cases where the global fixes have been applied but specific files still misbehave.

How to verify the fix worked

Open File Explorer to the affected folder. Wait 30 seconds for thumbnails to regenerate. They should appear normally.
Right-click empty space in the folder → View → Extra Large Icons. Forces detailed thumbnail generation.
For HEIC photos: thumbnails should show the actual image preview, not a generic camera icon.

If none of these work

If thumbnails are still black after installing extensions and rebuilding cache, a third-party shell extension is interfering. Test in Safe Mode: boot into Safe Mode (Settings → Recovery → Advanced startup → Restart now → Troubleshoot → Advanced → Startup Settings → Restart → 4). Open File Explorer to the problem folder. If thumbnails work in Safe Mode, a third-party extension is the cause — use ShellExView (NirSoft) to disable suspect extensions one at a time. For PCs with OneDrive Files On-Demand: cloud thumbnails take noticeably longer. If patience doesn’t help, mark folders as Always keep on this device. For Bitlocker-encrypted external drives: thumbnails generate slowly because Windows decrypts each file briefly. This is normal. For PCs with Windows Defender Real-time Protection: Defender scans each file as Explorer reads it for thumbnail generation. On low-end PCs this can fail with timeouts. Add the folder to Defender exclusion list temporarily to test — not as permanent fix but to confirm the cause.

Bottom line: Install HEIF/Raw extensions from Microsoft Store for format-specific issues, then rebuild thumbnail cache via Disk Cleanup’s Thumbnails category. Most black thumbnails are missing-codec issues, not cache corruption.