The MIDI.org file specification does not specify any file extension, just the formats that may be used within the file.

Ableton has this to say (rather ungrammatically):
"""
[Ableton] Live can import standard MIDI files in the three main formats (SMF0, SMF1 and SMF2) with either *.mid or *.smf file extensions.

[Ableton] Live exports MIDI files in the SMF0 format only, with the extension *.mid.

While MIDI files may have the same file extensions (*.mid or *.smf), from the outside, it's not possible to tell which format a specific MIDI file is actually using. This information is stored in its header chunk only and can be revealed by opening the file with a hex editor. The format of a MIDI file is initially defined by the device or software the file is created with.
"""
https://help.ableton.com/hc/en-us/articles/209068169-Understanding-MIDI-files


All of which strongly suggests that the file extension is a local convention, but that any particular application may or may not accept all three types of content (SFM0, SMF1 or SMF2).

So whether or not the keyboard will accept the renamed file depends upon the content.

I have not tried very hard, but I have so far failed to find any .smf files to compare with a .mid file.

Jazz relative beginner, starting at a much older age than was helpful.
AVL:MXE Linux; Windows 11
BIAB2025 Audiophile, a bunch of other software.
Kawai MP6, Ui24R, Focusrite Saffire Pro40 and Scarletts
.