Rendering a web page using content communicated to a browser application from a process running on a client

A computer program product for allowing a browser application running on a client device to access image files stored on the client device, the computer program product comprising a non-transitory computer-readable storage medium containing a markup language document from a web application associated with an online system, the web application configured to upload image files from the client device to a web server of the online system, the markup language document comprising:

• content items encoded in the markup language document, the markup language document including instructions that load a browser plug-in configured to access a portion of a file system of the client device where the image files are located that the browser application is restricted from accessing;
• and
• instructions that, when interpreted by the browser application executing on the client device, cause the browser application to:
• process the markup language document to load the browser plug-in;
• send, by the browser application, a request to the browser plug-in to access the portion of the file system of the client device that the browser application is restricted from accessing and upload one or more of the image files, the request including a first hash value that allows the browser plug-in to verify authenticity of a sender of the markup language document;
• verify, by the browser plug-in, the authenticity of the sender of the markup language document by:
• determining a second hash value based on information in the received markup language document, and
• comparing the first hash value to the second hash value, wherein a successful verification of the authenticity of the sender of the markup language document includes the first hash value matching the second hash value;
• responsive to successful verification of the authenticity of the sender of the markup language document, access, by the browser plug-in, the resource in the portion of the file system identified in the request to upload the one or more of the image files;
• receiving, by the browser application, information describing progress of uploading the one or more of the image files;
• and
• rendering, by the Internet browser application, a user interface element displaying the progress uploading the one or more image files.