added test cases

core/src/main/java/org/kiwix/kiwixmobile/core/utils/files/FileUtils.kt

@@ -291,7 +291,12 @@ object FileUtils {
     return null
   }
 
-  private fun getDecodedFileName(url: String?, src: String?): String =
+  /**
+   * Returns the file name from the url or src. In url it gets the file name from the last '/' and
+   * if it contains '.'. If the url is null then it'll get the file name from the last '/'.
+   * If the url and src doesn't exist it returns the empty string.
+   */
+  fun getDecodedFileName(url: String?, src: String?): String =
     url?.substringAfterLast("/", "")
       ?.takeIf { it.contains(".") }
       ?: src?.substringAfterLast("/", "")

core/src/test/java/org/kiwix/kiwixmobile/core/utils/files/FileUtilsTest.kt

@@ -88,4 +88,44 @@ class FileUtilsTest {
     every { mockFile.path } returns "$fileName$extension"
     every { mockFile.exists() } returns fileExists
   }
+
+  @Test
+  fun `test decode file name`() {
+    val fileName =
+      FileUtils.getDecodedFileName(
+        url = "https://kiwix.org/contributors/contributors_list.pdf",
+        src = null
+      )
+    assertThat(fileName).isEqualTo("contributors_list.pdf")
+  }
+
+  @Test
+  fun `test file name if extension doesn't exist`() {
+    val fileName = FileUtils.getDecodedFileName(url = "https://kiwix.org/contributors/", src = null)
+    assertThat(fileName).isEqualTo("")
+  }
+
+  @Test
+  fun `test file name if the url and src doesn't exist`() {
+    val fileName = FileUtils.getDecodedFileName(url = null, src = null)
+    assertThat(fileName).isEqualTo("")
+  }
+
+  @Test
+  fun `test file name if only file name exist`() {
+    val fileName = FileUtils.getDecodedFileName(src = "android_tutorials.pdf", url = null)
+    assertThat(fileName).isEqualTo("")
+  }
+
+  @Test
+  fun `test file name if url doesn't exist`() {
+    val fileName = FileUtils.getDecodedFileName(url = null, src = "/html/images/test.png")
+    assertThat(fileName).isEqualTo("test.png")
+  }
+
+  @Test
+  fun `test file name if url and src's extension doesn't exist`() {
+    val fileName = FileUtils.getDecodedFileName(url = null, src = "/html/images/")
+    assertThat(fileName).isEqualTo("")
+  }
 }