This encoder produce MPEG4 AAC+ wrapped into 3gp (very-very close to mp4) container!
Code:
c:\dvdrip\mp4box\mp4box.exe -info test.3gp
* Movie Info *
Timescale 600 - Duration 00:05:36.468
Fragmented File no - 1 track(s)
File Brand 3gp4 - version 0
File has root IOD
Scene PL 0xff - Graphics PL 0xff - OD PL 0xff
Visual PL: No visual capability required (0xff)
Audio PL: High Quality Audio Profile @ Level 2 (0x0f)
No streams included in root OD
Track * 1 Info - TrackID 1 - TimeScale 48000 - Duration 00:05:36.469
Media Info: Language "und" - Type "soun" - Sub Type "mp4a" - 7886 samples
MPEG-4 Config: Audio Stream - ObjectTypeIndication 0x40
MPEG-4 Audio SBR - 2 Channel(s) - SampleRate 24000 - SBR SampleRate 48000
Self-synchronized
so best matching extension is m4a or mp4 (not aac)