Added BackupType with BackupSerializer

app/src/main/kotlin/com/simplemobiletools/smsmessenger/models/BackupType.kt

@@ -0,0 +1,13 @@
+package com.simplemobiletools.smsmessenger.models
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+@Serializable
+enum class BackupType {
+    @SerialName("sms")
+    SMS,
+
+    @SerialName("mms")
+    MMS,
+}

app/src/main/kotlin/com/simplemobiletools/smsmessenger/models/MessagesBackup.kt

@@ -0,0 +1,23 @@
+package com.simplemobiletools.smsmessenger.models
+
+import kotlinx.serialization.DeserializationStrategy
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.*
+
+@Serializable(with = BackupSerializer::class)
+sealed class MessagesBackup() {
+    @SerialName("backupType")
+    abstract val backupType: BackupType
+}
+
+object BackupSerializer :
+    JsonContentPolymorphicSerializer<MessagesBackup>(MessagesBackup::class) {
+    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out MessagesBackup> {
+        return when (element.jsonObject["backupType"]?.jsonPrimitive?.content) {
+            "sms" -> SmsBackup.serializer()
+            "mms" -> MmsBackup.serializer()
+            else -> throw Exception("ERROR: No Serializer found. Serialization failed.")
+        }
+    }
+}

app/src/main/kotlin/com/simplemobiletools/smsmessenger/models/MmsBackup.kt

@@ -46,7 +46,9 @@ data class MmsBackup(
     val addresses: List<MmsAddress>,
     @SerializedName("parts")
     val parts: List<MmsPart>,
-) {
+
+    override val backupType: BackupType = BackupType.MMS,
+): MessagesBackup() {
 
     fun toContentValues(): ContentValues {
         return contentValuesOf(

app/src/main/kotlin/com/simplemobiletools/smsmessenger/models/SmsBackup.kt

@@ -30,8 +30,10 @@ data class SmsBackup(
     @SerializedName("type")
     val type: Int,
     @SerializedName("service_center")
-    val serviceCenter: String?
-) {
+    val serviceCenter: String?,
+
+    override val backupType: BackupType = BackupType.SMS,
+    ): MessagesBackup() {
 
     fun toContentValues(): ContentValues {
         return contentValuesOf(