diff --git a/scripts/importMysteryTemplate.ts b/scripts/importMysteryTemplate.ts
index ff63712..3982c49 100644
--- a/scripts/importMysteryTemplate.ts
+++ b/scripts/importMysteryTemplate.ts
@@ -41,11 +41,18 @@ function requireOk(response: Response, action: string) {
   return response.text().then(body => { throw new Error(`${action} failed (${response.status}): ${body}`) })
 }
 
+const MIME_BY_EXT: Record<string, string> = {
+  png: 'image/png', jpg: 'image/jpeg', jpeg: 'image/jpeg', gif: 'image/gif', webp: 'image/webp', svg: 'image/svg+xml',
+  pdf: 'application/pdf', mp3: 'audio/mpeg', wav: 'audio/wav', ogg: 'audio/ogg', m4a: 'audio/mp4', txt: 'text/plain',
+}
+function mimeFor(filename: string) { return MIME_BY_EXT[filename.split('.').pop()?.toLowerCase() || ''] || 'application/octet-stream' }
+
 async function uploadAsset(baseUrl: string, levelId: string, manifestDir: string, document: MysteryDocument, authorization?: string) {
   if (!document.asset) return undefined
   const assetPath = path.resolve(manifestDir, document.asset)
+  const filename = path.basename(assetPath)
   const form = new FormData()
-  form.append('file', new Blob([await readFile(assetPath)]), path.basename(assetPath))
+  form.append('file', new Blob([await readFile(assetPath)], { type: mimeFor(filename) }), filename)
   const response = await requireOk(await fetch(`${baseUrl}/api/levels/${levelId}/documents?edit=1`, { method: 'POST', headers: authorization ? { authorization } : undefined, body: form }), `Upload ${document.asset}`)
   return await response.json() as CaseDocument
 }