There are only three FTE template types for file entries:
As PML said, the Externalimslpfile template is deprecated. Its variable names are very strange, so I would advise against trying to parse it.
I think there are two things we can do for the iPad app:
1. ignore fte:server-us (these files are files PD only in the US), or
2. do a mass convert of Externalimslpfile into server-us and implement server-us; conversion maybe via a bot (the conversion is actually very simple, see below). Note that because the US server is not associated with the IMSLP site proper, US server files are not catalogued in the IMSLP index system. The download URL is also different.
For some background on what happened with server-us/externalimslpfile, and how to convert them (note that the bot must be logged in and send the correct login cookies):viewtopic.php?f=7&t=4716&p=24425#p24455
Note that, while most of the variable names are the same, server-us has |File Path= and |File Size= instead of |File Name=, and |Date Submitted= format is different.