Many web browsers have a form field auto-complete feature. This feature allows your web browser to "remember" data that you previously entered into a field on a web page (like an email address).
For most web browsers when this feature is enabled, you simply click on an empty form field and a list will appear and drop down with all of the possible values, which you entered previously (in this case, a list of email addresses to which you sent files previously).
If you do not have this feature turned on for your web browser, then you may want to consider doing this.
If you use Internet Explorer, then to learn more use a search engine to perform a search using the keywords: Internet Explorer AutoComplete
If you use Firefox, then to learn more use a search engine to perform a search using the keywords: Firefox Autofill