Index: src/bp-core/classes/class-bp-attachment.php
===================================================================
--- src/bp-core/classes/class-bp-attachment.php
+++ src/bp-core/classes/class-bp-attachment.php
@@ -268,6 +268,11 @@
 			add_filter( 'upload_dir', $upload_dir_filter, 10, $this->upload_dir_filter_args );
 		}
 
+		// Helper for utf-8 filenames.
+		if ( function_exists( 'iconv' ) ) {
+			add_filter( 'sanitize_file_name', array( $this, 'iconv' ) );
+		}
+
 		// Upload the attachment.
 		$this->attachment = wp_handle_upload( $file[ $this->file_input ], $overrides, $time );
 
@@ -276,6 +281,10 @@
 			remove_filter( 'upload_dir', $upload_dir_filter, 10 );
 		}
 
+		if ( function_exists( 'iconv' ) ) {
+			remove_filter( 'sanitize_file_name', array( $this, 'iconv' ) );
+		}
+
 		// Remove the pre WordPress 4.0 static filter.
 		remove_filter( 'wp_handle_upload_prefilter', array( $this, 'validate_upload' ), 10 );
 
@@ -284,6 +293,18 @@
 	}
 
 	/**
+	 * Helper to convert utf-8 characters in filenames to their ASCII equivalent.
+	 *
+	 * @since 2.9.0
+	 *
+	 * @param  string $retval Filename.
+	 * @return string
+	 */
+	public function iconv( $retval ) {
+		return iconv( 'UTF-8', 'ASCII//TRANSLIT//IGNORE', $retval );
+	}
+
+	/**
 	 * Validate the allowed mime types using WordPress allowed mime types.
 	 *
 	 * In case of a multisite, the mime types are already restricted by