File Upload and Media Processing#

Misskey's file upload pipeline runs through three main layers: MIME detection (what the file is), media processing (generating display-optimized variants), and Content-Type enforcement (controlling browser behavior at serve time). The primary entry point is DriveService.addFile, which orchestrates all three.

MIME Type Detection & Validation#

FileInfoService.detectType detects MIME type by reading magic bytes with the file-type library. Key edge cases handled:

• SVG: If file-type returns application/xml or is inconclusive, checkSvg runs an is-svg check (files >1 MB are rejected as SVG).
• Video-only audio: Files detected as video but lacking a video track (m4a, some webm) are re-typed to their audio/* counterpart via ffprobe 1.
• MIME normalization: audio/x-flac → audio/flac, audio/vnd.wave → audio/wav via fixMime.
• Zero-byte files and unrecognized files fall back to application/octet-stream 2.
• Filename-based fallback: .txt, .csv, .json extensions are applied if file-type returns octet-stream 3.

getFileInfo also rejects images with dimensions exceeding 16383 × 16383 px by downgrading them to octet-stream 4, and computes MD5 hash, image dimensions (via probe-image-size), and blurhash.

MIME Whitelists#

Two named whitelists live in const.ts:

The isMimeImage utility extends this with four named subsets:

Image Processing & Alt Generation#

DriveService.generateAlts produces up to two variants per upload:

Thumbnail — always generated for Sharp-convertible images and videos:

• Static images: WebP at 498 × 422 via convertSharpToWebp
• Animated images (GIF, WebP): WebP at 374 × 317 with alphaQuality: 70 5
• Videos: delegated to VideoProcessingService.generateVideoThumbnail, or skipped if config.videoThumbnailGenerator is set 6

Webpublic (web-optimized copy) — generated only for local uploads (!file.uri) when the original doesn't already meet web-safe criteria:

• JPEG/WebP/AVIF → WebP at 2048 × 2048 7
• PNG/BMP/SVG → PNG at 2048 × 2048 8
• Skipped if: image is animated, has EXIF/IPTC/XMP metadata and is ≤2048px, or is SVG/AVIF (security/compatibility) 9

All Sharp operations use sharpBmp (a Misskey-maintained wrapper that adds BMP/ICO support to Sharp). The default WebP encoding uses quality: 77, smartSubsample: true, effort: 2 10.

Both thumbnail and webpublic are stored separately — their URLs are tracked as thumbnailUrl/webpublicUrl on MiDriveFile and uploaded to object storage (or internal storage) in parallel with the original 11.

Content-Type Enforcement at Serve Time#

getSafeContentType enforces FILE_TYPE_BROWSERSAFE: any MIME not in the list is served as application/octet-stream, forcing a browser download instead of inline rendering.

This is applied in two places:

• FileServerUtils.setFileResponseHeaders: sets Content-Type via getSafeContentType for all local file responses 12.
• DriveService.upload (object storage upload): sets ContentType on the S3 PutObject call the same way; image/apng is also normalized to image/png at this point 13. For object storage, file extensions are also stripped from the key URL for non-browsersafe types to prevent content-type inference from the filename 14.

The proxy handler (FileServerProxyHandler) applies additional restrictions: SVG is converted to WebP 15, and non-image or non-browsersafe MIME types cause a 403 Rejected type 16.

Key Files#