diff --git a/include/Makefile.in b/include/Makefile.in
index 02ee45d9603..856d52d8ea5 100644
--- a/include/Makefile.in
+++ b/include/Makefile.in
@@ -626,6 +626,7 @@ SOURCES = \
 	tcpmib.h \
 	textserv.h \
 	textstor.idl \
+	thumbcache.idl \
 	timezoneapi.h \
 	tlhelp32.h \
 	tlogstg.idl \
diff --git a/include/thumbcache.idl b/include/thumbcache.idl
new file mode 100644
index 00000000000..f79d879ab07
--- /dev/null
+++ b/include/thumbcache.idl
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2019 Alistair Leslie-Hughes
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
+ */
+
+import "oaidl.idl";
+import "shtypes.idl";
+
+typedef [v1_enum] enum WTS_FLAGS
+{
+    WTS_NONE                 = 0x00000000,
+    WTS_EXTRACT              = 0x00000000,
+    WTS_INCACHEONLY          = 0x00000001,
+    WTS_FASTEXTRACT          = 0x00000002,
+    WTS_FORCEEXTRACTION      = 0x00000004,
+    WTS_SLOWRECLAIM          = 0x00000008,
+    WTS_EXTRACTDONOTCACHE    = 0x00000020,
+    WTS_SCALETOREQUESTEDSIZE = 0x00000040,
+    WTS_SKIPFASTEXTRACT      = 0x00000080,
+    WTS_EXTRACTINPROC        = 0x00000100,
+    WTS_CROPTOSQUARE         = 0x00000200,
+    WTS_INSTANCESURROGATE    = 0x00000400,
+    WTS_REQUIRESURROGATE     = 0x00000800,
+    WTS_APPSTYLE             = 0x00002000,
+    WTS_WIDETHUMBNAILS       = 0x00004000,
+    WTS_IDEALCACHESIZEONLY   = 0x00008000,
+    WTS_SCALEUP              = 0x00010000,
+} WTS_FLAGS;
+
+typedef [v1_enum] enum WTS_ALPHATYPE
+{
+    WTSAT_UNKNOWN = 0,
+    WTSAT_RGB     = 1,
+    WTSAT_ARGB    = 2
+} WTS_ALPHATYPE;