diff --git a/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/mapping/DomainMapping.kt b/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/mapping/DomainMapping.kt
index 133c069aaa4..344ec999ccc 100644
--- a/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/mapping/DomainMapping.kt
+++ b/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/mapping/DomainMapping.kt
@@ -585,7 +585,7 @@ internal class DomainMapping(
             authorName = author_name,
             authorLink = author_link,
             fallback = fallback,
-            fileSize = file_size,
+            fileSize = file_size ?: 0,
             image = image,
             imageUrl = image_url,
             mimeType = mime_type,
diff --git a/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/model/dto/AttachmentDto.kt b/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/model/dto/AttachmentDto.kt
index b9d3ca66c7f..a9965eda3eb 100644
--- a/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/model/dto/AttachmentDto.kt
+++ b/stream-chat-android-client/src/main/java/io/getstream/chat/android/client/api2/model/dto/AttachmentDto.kt
@@ -33,7 +33,7 @@ internal data class AttachmentDto(
     val author_name: String?,
     val author_link: String?,
     val fallback: String?,
-    val file_size: Int = 0,
+    val file_size: Int?,
     val image: String?,
     val image_url: String?,
     val mime_type: String?,
diff --git a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/Mother.kt b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/Mother.kt
index 8fccd64aabd..f61160b869a 100644
--- a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/Mother.kt
+++ b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/Mother.kt
@@ -750,7 +750,7 @@ internal object Mother {
         authorName: String? = randomString(),
         authorLink: String? = randomString(),
         fallback: String? = randomString(),
-        fileSize: Int = positiveRandomInt(),
+        fileSize: Int? = positiveRandomInt(),
         image: String? = randomString(),
         imageUrl: String? = randomString(),
         mimeType: String? = randomString(),
diff --git a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/api2/mapping/DomainMappingTest.kt b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/api2/mapping/DomainMappingTest.kt
index d4d4ba07ff5..e4740a9db82 100644
--- a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/api2/mapping/DomainMappingTest.kt
+++ b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/api2/mapping/DomainMappingTest.kt
@@ -558,7 +558,7 @@ internal class DomainMappingTest {
             authorName = attachmentDto.author_name,
             authorLink = attachmentDto.author_link,
             fallback = attachmentDto.fallback,
-            fileSize = attachmentDto.file_size,
+            fileSize = attachmentDto.file_size ?: 0,
             image = attachmentDto.image,
             imageUrl = attachmentDto.image_url,
             mimeType = attachmentDto.mime_type,
@@ -576,6 +576,16 @@ internal class DomainMappingTest {
         assertEquals(expected, attachment)
     }
 
+    @Test
+    fun `AttachmentDto with null file_size falls back to 0`() {
+        val attachmentDto = randomAttachmentDto(fileSize = null)
+        val sut = Fixture().get()
+        val attachment = with(sut) {
+            attachmentDto.toDomain()
+        }
+        assertEquals(0, attachment.fileSize)
+    }
+
     @Test
     fun `BannedUserResponse is correctly mapped to BannedUser`() {
         val bannedUserResponse = randomBannedUserResponse()
diff --git a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/AttachmentDtoAdapterTest.kt b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/AttachmentDtoAdapterTest.kt
index 61bc340ed4e..cb6367f1352 100644
--- a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/AttachmentDtoAdapterTest.kt
+++ b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/AttachmentDtoAdapterTest.kt
@@ -49,4 +49,10 @@ internal class AttachmentDtoAdapterTest {
         val jsonString = parser.toJson(AttachmentDtoTestData.attachmentWithoutExtraData)
         jsonString.shouldEqualJson(AttachmentDtoTestData.jsonWithoutExtraData)
     }
+
+    @Test
+    fun `Deserialize JSON attachment with null file_size`() {
+        val attachment = parser.fromJson(AttachmentDtoTestData.jsonWithNullFileSize, AttachmentDto::class.java)
+        attachment shouldBeEqualTo AttachmentDtoTestData.attachmentWithNullFileSize
+    }
 }
diff --git a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/testdata/AttachmentDtoTestData.kt b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/testdata/AttachmentDtoTestData.kt
index 20e1f11ffa8..0504b4bfd7b 100644
--- a/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/testdata/AttachmentDtoTestData.kt
+++ b/stream-chat-android-client/src/test/java/io/getstream/chat/android/client/parser2/testdata/AttachmentDtoTestData.kt
@@ -65,6 +65,33 @@ internal object AttachmentDtoTestData {
         extraData = mapOf("draft" to true),
     )
 
+    @Language("JSON")
+    val jsonWithNullFileSize =
+        """{
+          "file_size": null
+        }
+        """.withoutWhitespace()
+    val attachmentWithNullFileSize = AttachmentDto(
+        asset_url = null,
+        author_name = null,
+        author_link = null,
+        fallback = null,
+        file_size = null,
+        image = null,
+        image_url = null,
+        mime_type = null,
+        name = null,
+        og_scrape_url = null,
+        text = null,
+        thumb_url = null,
+        title = null,
+        title_link = null,
+        type = null,
+        original_width = null,
+        original_height = null,
+        extraData = emptyMap(),
+    )
+
     @Language("JSON")
     val jsonWithoutExtraData =
         """{