The IRS website tells me that I can file my tax forms electronically at https://www.freefilefillableforms.com/. However, I think I will need to attach some additional documents in order to complete my tax filing. Is there a way to do this?
You cannot attach documents; but you don't need to.
The IRS assumes all data entered in the forms is correct, and you have the respective documents; your tax is decided on that basis.
Only if they select you for an audit, they will contact you, and then you need the recipts, W2s, etc. So keep all orignals secure (for 6 years), and file your taxes without attachments.
You cannot attach documents on free fillable forms, except those which are built into the program.
From the website:
General Program Limitations
Attaching Statements- This program does not allow you to attach any documents to your return, except those available through the Add/View Forms menu. If you need to attach any such documents, you will have to print and mail in the return.
However, most standard forms are available. Form W-2 for example allows you to enter the W-2 information; Form 8949 allows you to report sale of capital assets (such as stocks); Form 2106 allows for reporting Employee Business Expenses; etc. In most cases, nothing beyond that needs to be sent into the IRS. (Note: the W-2 itself doesn't need to be sent in, in any event - the IRS already has that from the employer, you're just using it for the information.) The full list of forms and schedules available is on that same web page.
If it turns out you do need to send things in to the IRS, you will need to mail in your return (which you can do from the Free Fillable Forms - it gives you a full 1040 at the end which you can then mail in). My suggestion would be to fill in the forms, and then if you get to somewhere that the instructions say to mail in something not available through the program, finish as best you can and mail in the return.