Firefox Plugin to modify attachment on gmail right before upload.

I need a Firefox plugin that can modify an attachment (from lowercase letters to uppercase letters) on Gmail compose box right before uploading. One way could be to replace the attachment button with our own attachment button through Javascript, so that when the button is clicked, the file selection window pops up and when the user selects the file, the script should modify the file before Gmail could upload them. I found following steps which are reliable but with 1 small missing link: /** * 1) Create a new attachment button, and hide the existing button. * 2) When this button is clicked, create a new hidden file input and trigger click on it to * prompt the user to select the files they want to upload; add a change listener to this input to know * when files have been selected. * 3) Get that list of files, and perform encryption on them. * 4) Create a new div element with the expected '_hfn' id, where n is an integer starting at 1, * incrementing based on (the number of files attached when this reply pane was opened? Number of * files currently attached? An internal counter? Some other factor?). Attach * an array containing the altered files to this element as its 'files' property. * 5) Add a click listener to this element, so that when it receives the click event it, on a timer, * will trigger the change event on itself, which should in turn cause the gmail api to grab the * files array from it and perform the upload as usual. */ Another way could be to figure out the HTTP headers that the Gmail sends as request to Google's server in order to upload the file and modify the contents of the HTTP headers. The HTTP headers can be either intercepted and modified right before they are sent, or they the attachment button can be replaced with our own button so that when the user clicks on it, our own HTTP request is sent to Google server which looks similar to Gmail's but with modified file's content. Please do some research on this before applying for the project. Thanks.