Use optipng to optimise document thumbnails

src/paperless/checks.py

@@ -76,7 +76,12 @@ def binaries_check(app_configs, **kwargs):
     error = "Paperless can't find {}. Without it, consumption is impossible."
     hint = "Either it's not in your ${PATH} or it's not installed."
 
-    binaries = (settings.CONVERT_BINARY, settings.UNPAPER_BINARY, "tesseract")
+    binaries = (
+        settings.CONVERT_BINARY,
+        settings.OPTIPNG_BINARY,
+        settings.UNPAPER_BINARY,
+        "tesseract"
+    )
 
     check_messages = []
     for binary in binaries:

src/paperless/settings.py

@@ -247,6 +247,9 @@ CONVERT_TMPDIR = os.getenv("PAPERLESS_CONVERT_TMPDIR")
 CONVERT_MEMORY_LIMIT = os.getenv("PAPERLESS_CONVERT_MEMORY_LIMIT")
 CONVERT_DENSITY = os.getenv("PAPERLESS_CONVERT_DENSITY")
 
+# OptiPNG
+OPTIPNG_BINARY = os.getenv("PAPERLESS_OPTIPNG_BINARY", "optipng")
+
 # Unpaper
 UNPAPER_BINARY = os.getenv("PAPERLESS_UNPAPER_BINARY", "unpaper")