Fix some issues when reading/saving images (#993)

* Fix unsupported mime type error when saving images

Avoid using platform mime type map to get extensions as it may not have
all mime types we support.

* Fix jxl images downloading/reading

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

@@ -17,6 +17,7 @@ import logcat.LogPriority
 import tachiyomi.core.common.i18n.stringResource
 import tachiyomi.core.common.storage.extension
 import tachiyomi.core.common.util.lang.launchIO
+import tachiyomi.core.common.util.system.ImageUtil
 import tachiyomi.core.common.util.system.logcat
 import tachiyomi.domain.category.interactor.GetCategories
 import tachiyomi.domain.chapter.model.Chapter
@@ -160,7 +161,7 @@ class DownloadManager(
     fun buildPageList(source: Source, manga: Manga, chapter: Chapter): List<Page> {
         val chapterDir = provider.findChapterDir(chapter.name, chapter.scanlator, manga.title, source)
         val files = chapterDir?.listFiles().orEmpty()
-            .filter { "image" in it.type.orEmpty() }
+            .filter { it.isFile && ImageUtil.isImage(it.name) { it.openInputStream() } }
 
         if (files.isEmpty()) {
             throw Exception(context.stringResource(MR.strings.page_list_empty_error))

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

@@ -523,14 +523,8 @@ class Downloader(
      * @param file the file where the image is already downloaded.
      */
     private fun getImageExtension(response: Response, file: UniFile): String {
-        // Read content type if available.
         val mime = response.body.contentType()?.run { if (type == "image") "image/$subtype" else null }
-            // Else guess from the uri.
-            ?: context.contentResolver.getType(file.uri)
-            // Else read magic numbers.
-            ?: ImageUtil.findImageType { file.openInputStream() }?.mime
-
-        return ImageUtil.getExtensionFromMimeType(mime)
+        return ImageUtil.getExtensionFromMimeType(mime) { file.openInputStream() }
     }
 
     private fun splitTallImageIfNeeded(page: Page, tmpDir: UniFile) {