use name as alt, display max size

src/db.ml

@@ -35,14 +35,18 @@ let () =
     Format.eprintf "db error@\n";
     exit 1
 
-let mime_database = Conan.Process.database ~tree:Conan_light.tree
-
-let mime contents =
-  match Conan_string.run ~database:mime_database contents with
+let mime =
+  let database = Conan.Process.database ~tree:Conan_light.tree in
+  fun content ->
+    match Conan_string.run ~database content with
     | Ok m -> Conan.Metadata.mime m
     | Error _ -> None
 
 let clean_image image =
+  let max_name = 1000 in
+  let max_alt = 3000 in
+  let max_content = 4200000 in
+
   let name, alt, content = image in
   let name =
     match name with
@@ -51,9 +55,13 @@ let clean_image image =
       (* make up random name if no name was given *)
       Uuidm.to_string (Uuidm.v4_gen random_state ())
   in
-  if String.length name > 1000 then Error "Image name too long"
-  else if String.length alt > 1000 then Error "Image description too long"
-  else if String.length content > 4200000 then Error "Image size too big"
+  let alt = if String.trim alt = "" then name else alt in
+  if String.length name > max_name then
+    Error (Format.sprintf "Image name too long: More than %dB" max_name)
+  else if String.length alt > max_alt then
+    Error (Format.sprintf "Image description too long: More than %dB" max_alt)
+  else if String.length content > max_content then
+    Error (Format.sprintf "Image size too big: More than %dB" max_content)
   else
     match mime content with
     | None -> Error "invalid image type"