Single files are files outside of a disk image, typically using a TIFILES header. They are also called FIADs ("file in a directory"), and each one is a file of the TI file system - stored outside of that file system but on a PC. The particular file type is declared inside the TIFILES header; for more detail information see the Ninerpedia entry.
Single files are not directly supported in MAME, which is mostly explained by the emulation concept of MAME, where the disk image is precisely emulated with all particular meta-information. Moreover, MAME is not supposed to invent new hardware, and the old hardware - the disk controllers - assume that the drive reads a floppy disk (or hard disk).
Unlike MAME, the popular Classic99 emulator make extensive use of the single files, as its emulation concept also includes new kinds of devices to overcome old limitations. Here, the storage DSRs are specially designed to work with those files on a PC drive.
So during the last years, the answer would be no to the question whether MAME can use these FIADs. A solution showed up with the introduction of the TIPI emulation: If you put the files on the TIPI device, MAME will be able to work with them, since all handling is done by TIPI.