Check whether text attachment is UTF-8

app/src/main/java/eu/faircode/email/CharsetHelper.java

@@ -23,6 +23,9 @@ import android.text.TextUtils;
 import android.util.Pair;
 
 import java.io.BufferedInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.charset.CharacterCodingException;
@@ -76,6 +79,19 @@ public class CharsetHelper {
         return isUTF8(octets);
     }
 
+    static boolean isUTF8(File file) {
+        try (FileInputStream fis = new FileInputStream(file)) {
+            ByteArrayOutputStream os = new ByteArrayOutputStream(Math.max(MAX_SAMPLE_SIZE, fis.available()));
+            byte[] buffer = new byte[MAX_SAMPLE_SIZE];
+            for (int len = fis.read(buffer); len != -1; len = fis.read(buffer))
+                os.write(buffer, 0, len);
+            return isUTF8(os.toByteArray());
+        } catch (Throwable ex) {
+            Log.e(ex);
+            return false;
+        }
+    }
+
     static boolean isUTF8(byte[] octets) {
         return isValid(octets, StandardCharsets.UTF_8);
     }

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -1473,13 +1473,14 @@ public class MessageHelper {
                     BodyPart attachmentPart = new MimeBodyPart();
 
                     File file = attachment.getFile(context);
+                    boolean isUtf8 = (attachment.type != null && attachment.type.startsWith("text/") && CharsetHelper.isUTF8(file));
 
                     FileDataSource dataSource = new FileDataSource(file);
                     dataSource.setFileTypeMap(new FileTypeMap() {
                         @Override
                         public String getContentType(File file) {
                             // https://tools.ietf.org/html/rfc6047
-                            if ("text/calendar".equals(attachment.type))
+                            if ("text/calendar".equals(attachment.type) || isUtf8)
                                 return attachment.type + "; charset=UTF-8;";
 
                             return attachment.type;