You can actually use the create a link button in the Description field editor, and that gives you an option to link to a file uploaded to the moodle course's file area. You can upload a file through this dialog box too:
1) Create an assignment (Upload a Single File if the assignment is just for one file to be submitted by each student)
2) In the Description field, enter the assignment instructions. Select the text which is to be the link to the file (e.g. worksheet as you suggest), then click the Insert Web Link button (the one which is the picture of a chain just above the white text area).
3) Click the Browse button, which loads up another dialog box showing a list of files in that course. Click the Browse button bottom left, and the teacher can then select the worksheet file.
4) Click the Upload button, then the window will refresh showing this new file in the list
5) Click the name of that file (NOT the checkbox next to the name - i.e. the underlined bit of the file name) and that box will close showing the link to the file.
6) Give the link a title (i.e. a short description such as 'worksheet for assignment name here') then click OK.
There is now a link in the assignment description to the worksheet.
Hope that helps.