As discussed in Nolo's article "How to Get a Copy of Your Immigration File (FOIA Requests)," applicants for immigration benefits might want to find out what the U.S. immigration authorities already have on file for them for any number of reasons. Previous applications may have gone astray within the immigration system, or the immigrant him- or herself might have lost important documents such as an I-130 approval notice.
In the past, getting a copy of one's immigration file involved filling out and then mailing, emailing, or faxing it. But while that old process still exists, a newer, easier alternative has been developed: Applicants can go through the Department of Homeland Security (DHS) website. It offers a "DHS FOIA Request Submission Form," which allows you to choose USCIS or another immigration-related agency from a drop-down menu, fill in other information, and submit the request online.
(The request itself is free, though you may have to pay for copying if your file is particularly large.)
Hopefully this will speed up a process that can sometimes go painfully slowly.