Charset improvements

FAQ.md

@@ -718,6 +718,9 @@ Account permissions are required to periodically refresh the [OAuth](https://dev
 (a kind of password used to login to your Gmail account).
 Just start the wizard (but do not select an account) to grant the required permissions again.
 
+The warning *... Unsupported encoding ...* means that the character set of the message is unknown or not supported.
+FairEmail will assume ISO-8859-1 (Latin1), which will in most cases result in showing the message correctly.
+
 See [here](https://linux.die.net/man/3/connect) for what error codes like EHOSTUNREACH and ETIMEDOUT mean.
 
 Possible causes are:

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

@@ -882,15 +882,19 @@ public class MessageHelper {
                 if (charset != null) {
                     charset = charset.replace("\"", "");
                     if ("ASCII".equals(charset.toUpperCase()))
-                        charset = "us-ascii";
+                        charset = "US-ASCII";
                 }
 
                 if (TextUtils.isEmpty(charset) || "US-ASCII".equals(charset.toUpperCase())) {
                     // The first 127 characters are the same as in US-ASCII
                     result = new String(result.getBytes(StandardCharsets.ISO_8859_1));
                 } else {
-                    if ("US-ASCII".equals(Charset.forName(charset).name()))
+                    // See UnknownCharsetProvider class
+                    if ("US-ASCII".equals(Charset.forName(charset).name())) {
+                        Log.w("Unsupported encoding charset=" + charset);
                         warnings.add(context.getString(R.string.title_no_charset, charset));
+                        result = new String(result.getBytes(StandardCharsets.ISO_8859_1));
+                    }
                 }
             } catch (ParseException ex) {
                 Log.w(ex);

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

@@ -20,6 +20,7 @@ package eu.faircode.email;
 */
 
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.nio.charset.spi.CharsetProvider;
 import java.util.Collections;
 import java.util.Iterator;
@@ -36,8 +37,9 @@ public class UnknownCharsetProvider extends CharsetProvider {
         // UseInqueCodePage
         // none
         // unknown-8bit
+        // X-UNKNOWN
         // https://javaee.github.io/javamail/FAQ#unsupen
-        Log.w("Unknown charset=" + name);
-        return Charset.forName("US-ASCII");
+        Log.e("Unknown charset=" + name);
+        return StandardCharsets.US_ASCII;
     }
 }