add integration with posts creating

features/content/common/src/commonMain/kotlin/dev/inmo/postssystem/features/content/common/Content.kt

@@ -3,6 +3,7 @@ package dev.inmo.postssystem.features.content.common
 import dev.inmo.micro_utils.common.FileName
 import dev.inmo.micro_utils.mime_types.MimeType
 import dev.inmo.postssystem.features.common.common.SimpleInputProvider
+import kotlinx.serialization.PolymorphicSerializer
 import kotlinx.serialization.Serializable
 import kotlin.jvm.JvmInline
 
@@ -26,11 +27,13 @@ interface SimpleContent : Content
 /**
  * This type represents some binary data which can be sent with multipart and deserialized from it
  */
-interface BinaryContent : Content {
-    val filename: FileName
-    val mimeType: MimeType
+data class BinaryContent(
+    val filename: FileName,
+    val mimeType: MimeType,
     val inputProvider: SimpleInputProvider
-}
+) : Content
+
+val ContentSerializer = PolymorphicSerializer(Content::class)
 
 /**
  * Content which is already registered in database. Using its [id] you can retrieve all known