You may find many ways to upload attachments in Salesforce using visualforce however most of them uses some Javascript libraries (means either you need to depend on static resources or add CDN in remote site settings) or they do not have progress bar or they are not drag and drop. I thought to create one simple Visualforce component which can show progress bar and have drag and drop capabilities.
Initially i gave though to use visualforce remoting however challenge was how to show progress bar and same challenge was with custom controller or extension also. So, i decided to create one simple Apex REST API to upload attachment.
