Shamim's Plugins

I understand you. But this plugin support multiple recipients. Suppose you have started a conversation where recipients are 5 persons. So you are sender but receivers are 5 persons. So which users avatar will show?

I can not access your Back end or message system. Please log in with this user and access message system or back end. You will see error.

Also if possible change language to english for testing.

Please check following
1. Allow attachment in Front End PM PRO Settings
2. File mime type is present and it is in allowed type
3. File size is less then from Front End PM PRO Settings

You can also remove some check from this function and try to find out which check failed. eg remove

if( !$mime || !in_array( $mime, get_allowed_mime_types() ) )
	return false;

then try. Also remove other check and try.

I am not sure if i will add in future because it is not most of the users use and it need a pretty big library. Also it use unnecessary resources for our purposes.

But time will tell if anytime need this to include.

Please use following (Untested).
let me know

function action_putmessage( $args ) {
	global $out, $admin_user_login;
	
	if( ! function_exists( 'fep_send_message' ) )
	return false;
	
	$sender = fep_get_userdata( $admin_user_login, 'ID', 'login' );
	
	if (!$sender) {
		$out['errmsg'] = "No such sender '$admin_user_login'";
		return false;
	}
	$recipient = fep_get_userdata( $args['user'], 'ID', 'login' );
	if (!$recipient) {
		$out['errmsg'] = "No such recipient '{$args['user']}'";
		return false;
	}
	$message = array(
		'message_title' => $args['title'],
		'message_content' => $args['message'],
		'message_to_id' => $recipient,
		'open_emr_args' => $args, //pass to get in attachment function
	);
	$override = array(
		'post_author' => $sender,
	);
	$message_id = fep_send_message( $message, $override );
}

add_action ('fep_action_message_after_send', 'fep_open_emr_upload_attachments', 10, 3 );

function fep_open_emr_upload_attachments( $message_id, $message, $inserted_message ){
	if ( ! fep_get_option( 'allow_attachment', 1 ) || ! $message_id || empty( $message['open_emr_args']) )
	return false;
	
	$args = $message['open_emr_args'];
	
	$name = isset( $args['filename'] ) ? $args['filename'] : '';
	
	if( ! $name )
	return false;
	
	$size_limit = (int) wp_convert_hr_to_bytes(fep_get_option('attachment_size','4MB'));
	$fields = (int) fep_get_option('attachment_no', 4);
	
	if( class_exists( 'Fep_Attachment' ) ){
		add_filter('upload_dir', array(Fep_Attachment::init(), 'upload_dir'));
	}
	
	$mime = isset( $args['mimetype'] ) ? $args['mimetype'] : '';
	$content = isset( $args['contents'] ) ? base64_decode($args['contents']) : '';
	
	if( !$mime || !in_array( $mime, get_allowed_mime_types() ) )
	return false;
	
	$size = strlen( $content );
	if( $size > $size_limit )
	return false;
	
	$att = wp_upload_bits( $name, null, $content );
	
	if( ! isset( $att['file'] ) || ! isset( $att['url'] ) || ! isset( $att['type'] ) )
	return false;
	
	$attachment = array(
		'guid'           => $att['url'], 
		'post_mime_type' => $att['type'],
		'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $att['url'] ) ),
		'post_content'   => '',
		'post_author'	=> $inserted_message->post_author,
		'post_status'    => 'inherit'
	);
	
	// Insert the attachment.
	wp_insert_attachment( $attachment, $att['file'], $message_id );
	
	if( class_exists( 'Fep_Attachment' ) ){
		remove_filter('upload_dir', array(Fep_Attachment::init(), 'upload_dir'));
	}
}

Please let me know following so that i can write some code for you.

1. Sender is always $admin_user_login?
2. what is $args[‘user’] ? ( user id or login ? )
3. maximum 1 attachment allowed?
4. Should i have to check mime type, size etc or it is already validated?

You can use https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_new_message_form/
This will show a form. when user send message it will use Front End PM PRO message system.

PRO version and free version use same text domain. So which string present in both version only those string translated. others you have to translate by using pot file supplied with PRO version.