Use custom mime discovery in downloader

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

@@ -14,10 +14,7 @@ import eu.kanade.tachiyomi.source.SourceManager
 import eu.kanade.tachiyomi.source.model.Page
 import eu.kanade.tachiyomi.source.online.HttpSource
 import eu.kanade.tachiyomi.source.online.fetchAllImageUrlsFromPageList
-import eu.kanade.tachiyomi.util.DynamicConcurrentMergeOperator
-import eu.kanade.tachiyomi.util.RetryWithDelay
-import eu.kanade.tachiyomi.util.plusAssign
-import eu.kanade.tachiyomi.util.saveTo
+import eu.kanade.tachiyomi.util.*
 import okhttp3.Response
 import rx.Observable
 import rx.android.schedulers.AndroidSchedulers
@@ -26,7 +23,6 @@ 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.
@@ -407,9 +403,7 @@ class Downloader(private val context: Context, private val provider: DownloadPro
             // Else guess from the uri.
             ?: context.contentResolver.getType(file.uri)
             // Else read magic numbers.
-            ?: file.openInputStream().buffered().use {
-            URLConnection.guessContentTypeFromStream(it)
-        }
+            ?: DiskUtil.findImageMime { file.openInputStream() }
 
         return MimeTypeMap.getSingleton().getExtensionFromMimeType(mime) ?: "jpg"
     }