diff --git a/app-ios/tutanota/Sources/CustomSchemeHandlers.swift b/app-ios/tutanota/Sources/CustomSchemeHandlers.swift
index 4e46c03dfb25..7582ff927c24 100644
--- a/app-ios/tutanota/Sources/CustomSchemeHandlers.swift
+++ b/app-ios/tutanota/Sources/CustomSchemeHandlers.swift
@@ -113,6 +113,8 @@ class AssetSchemeHandler: NSObject, WKURLSchemeHandler {
 			return "application/application/vnd.iccprofile"
 		} else if let mimeType = getFileMIMEType(path: path) {
 			return mimeType
+		} else if path.hasSuffix(".cmap") {
+			return "text/plain"  // used for invoices; no good mime type for cmap, so just use plain text
 		} else {
 			fatalError("Unknown asset type! \(path)")
 		}