Detect how to display pdf files

Closes: #1099

modules/webapp/src/main/elm/Data/Pdf.elm

@@ -0,0 +1,67 @@
+module Data.Pdf exposing (PdfMode(..), allModes, asString, detectUrl, fromString, serverUrl)
+
+{-| Makes use of the fact, that docspell uses a `/view` suffix on the
+path to provide a browser independent PDF view.
+-}
+
+import Data.Flags exposing (Flags)
+import Html exposing (..)
+import Html.Attributes exposing (..)
+
+
+type PdfMode
+    = Detect
+    | Native
+    | Server
+
+
+allModes : List PdfMode
+allModes =
+    [ Detect, Native, Server ]
+
+
+asString : PdfMode -> String
+asString mode =
+    case mode of
+        Detect ->
+            "detect"
+
+        Native ->
+            "native"
+
+        Server ->
+            "server"
+
+
+fromString : String -> Maybe PdfMode
+fromString str =
+    case String.toLower str of
+        "detect" ->
+            Just Detect
+
+        "native" ->
+            Just Native
+
+        "server" ->
+            Just Server
+
+        _ ->
+            Nothing
+
+
+serverUrl : String -> String
+serverUrl url =
+    if String.endsWith "/" url then
+        url ++ "view"
+
+    else
+        url ++ "/view"
+
+
+detectUrl : Flags -> String -> String
+detectUrl flags url =
+    if flags.pdfSupported then
+        url
+
+    else
+        serverUrl url