First, you need to create your form.
Go to Insert -> Form -> Form to add the form tag.
Go to Insert -> Form -> Text Field to add a text box. set the name of the text box to "FileName" (without the quotes)
Go to Insert -> Form -> File Field to add the file field. Set the name to "selectedFile" (no quotes again)
Go to Insert -> Form -> Button to insert the submit button.
Once the forn is created, add the upload file server behavior. Go to Insert -> WebAssist -> File Manipulation -> Upload.
Set the trigger to any form post.
Select the selectedFile file field.
On the second page, click the lightning bolt next to file name and select the FileName form field.
when filling out the form, type the name of the file in the file name form field, do not include the extension.
The extension will depend on the file that is uploaded, if the uploaded file is a jpg, the extension will be jpg, if it is an flv, the extension will be flv.