diff --git a/services/thumbnails/pkg/thumbnail/mimetypes.go b/services/thumbnails/pkg/thumbnail/mimetypes.go
deleted file mode 100644
index aff72a1d6b..0000000000
--- a/services/thumbnails/pkg/thumbnail/mimetypes.go
+++ /dev/null
@@ -1,23 +0,0 @@
-package thumbnail
-
-// SupportedMimeTypes are the types a preview is announced and attempted for
-// without further configuration. Both builds read all of them, webp included:
-// the pure Go build decodes it through the imaging library and answers the JPEG
-// the type mapping asks for. A generator that reads more than this is told so
-// with OC_THUMBNAIL_SOURCE_TYPES.
-var SupportedMimeTypes = map[string]struct{}{
-	"image/png":                         {},
-	"image/jpg":                         {},
-	"image/jpeg":                        {},
-	"image/gif":                         {},
-	"image/bmp":                         {},
-	"image/x-ms-bmp":                    {},
-	"image/tiff":                        {},
-	"image/webp":                        {},
-	"text/plain":                        {},
-	"audio/flac":                        {},
-	"audio/mpeg":                        {},
-	"audio/ogg":                         {},
-	"application/vnd.geogebra.slides":   {},
-	"application/vnd.geogebra.pinboard": {},
-}
diff --git a/services/webdav/pkg/thumbnail/sourcetypes.go b/services/webdav/pkg/thumbnail/sourcetypes.go
index 461c96ef28..6c4f275cf2 100644
--- a/services/webdav/pkg/thumbnail/sourcetypes.go
+++ b/services/webdav/pkg/thumbnail/sourcetypes.go
@@ -3,15 +3,33 @@ package thumbnail
 import (
 	"mime"
 	"strings"
-
-	thumbnails "github.com/opencloud-eu/opencloud/services/thumbnails/pkg/thumbnail"
 )
 
-// SourceTypes are the types a preview is announced and attempted for. The
-// defaults are the ones the thumbnails service compiles in, which is also where
-// reva reads them for oc:has-preview, so a build with libvips brings its extra
-// types along. Configured entries are merged on top here, per service: nobody
-// writes into that package's map.
+// DefaultSourceTypes are the types a preview is announced and attempted for
+// without further configuration. The image types are what both resize backends
+// read; the others are converted to an image here, by a preprocessor, before a
+// generator ever sees them.
+var DefaultSourceTypes = []string{
+	"image/png",
+	"image/jpg",
+	"image/jpeg",
+	"image/gif",
+	"image/bmp",
+	"image/x-ms-bmp",
+	"image/tiff",
+	"image/webp",
+	"text/plain",
+	"audio/flac",
+	"audio/mpeg",
+	"audio/ogg",
+	"application/vnd.geogebra.slides",
+	"application/vnd.geogebra.pinboard",
+}
+
+// SourceTypes are the types a preview is announced and attempted for: the
+// defaults plus what a deployment configured, which is also the list the
+// frontend hands to ocdav for oc:has-preview and graph answers its thumbnails
+// relationship from.
 //
 // The list is a hint, not a verdict. The generator decides what it can really
 // read, and a source it refuses answers 404.
@@ -19,13 +37,10 @@ type SourceTypes struct {
 	byType map[string]struct{}
 }
 
-// NewSourceTypes merges the configured types onto the compiled-in defaults.
+// NewSourceTypes merges the configured types onto the defaults.
 func NewSourceTypes(entries []string) SourceTypes {
-	t := SourceTypes{byType: make(map[string]struct{}, len(thumbnails.SupportedMimeTypes)+len(entries))}
-	for mimeType := range thumbnails.SupportedMimeTypes {
-		t.byType[mimeType] = struct{}{}
-	}
-	for _, entry := range entries {
+	t := SourceTypes{byType: make(map[string]struct{}, len(DefaultSourceTypes)+len(entries))}
+	for _, entry := range append(append([]string{}, DefaultSourceTypes...), entries...) {
 		if entry = strings.ToLower(strings.TrimSpace(entry)); entry != "" {
 			t.byType[entry] = struct{}{}
 		}
diff --git a/services/webdav/pkg/thumbnail/typemapping.go b/services/webdav/pkg/thumbnail/typemapping.go
index 335b83f87e..dc7eefbd1a 100644
--- a/services/webdav/pkg/thumbnail/typemapping.go
+++ b/services/webdav/pkg/thumbnail/typemapping.go
@@ -6,7 +6,7 @@ import (
 	"github.com/opencloud-eu/opencloud/services/webdav/pkg/generator"
 )
 
-// DefaultTypeMapping answers for every type in SupportedMimeTypes, so the
+// DefaultTypeMapping answers for every type in DefaultSourceTypes, so the
 // type a thumbnail is returned in can be read off here instead of being
 // guessed from a file name. Types that carry alpha or animation keep their own
 // format, everything else becomes a JPEG. An icon is the one type no generator