Revert "Support all image types (resolves #192)"

This reverts commit f67e4e6cb9.

src/library/entry.cr

@@ -40,7 +40,8 @@ class Entry
 
     file = ArchiveFile.new @zip_path
     @pages = file.entries.count do |e|
-      filename_is_image e.filename
+      SUPPORTED_IMG_TYPES.includes? \
+        MIME.from_filename? e.filename
     end
     file.close
   end
@@ -102,7 +103,8 @@ class Entry
     ArchiveFile.open @zip_path do |file|
       entries = file.entries
         .select { |e|
-          filename_is_image e.filename
+          SUPPORTED_IMG_TYPES.includes? \
+            MIME.from_filename? e.filename
         }
         .sort! { |a, b|
           compare_numerically a.filename, b.filename

src/library/types.cr

@@ -1,3 +1,5 @@
+SUPPORTED_IMG_TYPES = ["image/jpeg", "image/png", "image/webp"]
+
 enum SortMethod
   Auto
   Title