Improve extension discovery. Fix #542

app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt

@@ -25,6 +25,7 @@ import rx.subjects.BehaviorSubject
 import rx.subscriptions.CompositeSubscription
 import timber.log.Timber
 import uy.kohesive.injekt.injectLazy
+import java.net.URLConnection
 
 /**
  * This class is the one in charge of downloading chapters.
@@ -366,19 +367,22 @@ class Downloader(private val context: Context, private val provider: DownloadPro
 
     /**
      * Returns the extension of the downloaded image from the network response, or if it's null,
-     * analyze the file. If both fail, assume it's a jpg.
+     * analyze the file. If everything fails, assume it's a jpg.
      *
      * @param response the network response of the image.
      * @param file the file where the image is already downloaded.
      */
     private fun getImageExtension(response: Response, file: UniFile): String {
-        val contentType = response.body().contentType()
-        val mimeStr = if (contentType != null) {
-            "${contentType.type()}/${contentType.subtype()}"
-        } else {
-            context.contentResolver.getType(file.uri)
+        // Read content type if available.
+        val mime = response.body().contentType()?.let { ct -> "${ct.type()}/${ct.subtype()}" }
+        // Else guess from the uri.
+        ?: context.contentResolver.getType(file.uri)
+        // Else read magic numbers.
+        ?: file.openInputStream().buffered().use {
+            URLConnection.guessContentTypeFromStream(it)
         }
-        return MimeTypeMap.getSingleton().getExtensionFromMimeType(mimeStr) ?: "jpg"
+
+        return MimeTypeMap.getSingleton().getExtensionFromMimeType(mime) ?: "jpg"
     }
 
     /**

app/src/main/java/eu/kanade/tachiyomi/ui/reader/ReaderPresenter.kt

@@ -2,6 +2,7 @@ package eu.kanade.tachiyomi.ui.reader
 
 import android.os.Bundle
 import android.os.Environment
+import android.webkit.MimeTypeMap
 import eu.kanade.tachiyomi.R
 import eu.kanade.tachiyomi.data.cache.ChapterCache
 import eu.kanade.tachiyomi.data.cache.CoverCache
@@ -29,6 +30,7 @@ import rx.schedulers.Schedulers
 import timber.log.Timber
 import uy.kohesive.injekt.injectLazy
 import java.io.File
+import java.net.URLConnection
 import java.util.*
 
 /**
@@ -563,14 +565,21 @@ class ReaderPresenter : BasePresenter<ReaderActivity>() {
         // Copy file in background.
         Observable
                 .fromCallable {
-                    // File where the image will be saved.
+                    // Folder where the image will be saved.
                     val destDir = File(pictureDirectory)
                     destDir.mkdirs()
 
-                    val destFile = File(destDir, manga.title + " - " + chapter.name +
-                            " - " + (page.index + 1))
+                    // Find out file extension.
+                    val mime = context.contentResolver.getType(page.uri)
+                    ?: context.contentResolver.openInputStream(page.uri).buffered().use {
+                        URLConnection.guessContentTypeFromStream(it)
+                    }
+                    val ext = MimeTypeMap.getSingleton().getExtensionFromMimeType(mime) ?: "jpg"
+
+                    // Destination file.
+                    val destFile = File(destDir, manga.title + " - " + chapter.name +
+                            " - " + (page.index + 1) + ".$ext")
 
-                    // Location of image file.
                     context.contentResolver.openInputStream(page.uri).use { input ->
                         destFile.outputStream().use { output ->
                             input.copyTo(output)