If you upload an image with an EXIF GPSAltitude, and then fetch imageinfo via the API, you just get the difficult-to-work-with "rational" value, such as "9395/128". Many other fields, such as FocalLength, GPSTimestamp, etc. are displayed as rationals.
However GPSLatitude and GPSLongitude are shown with the expected "decimal" values.
Example: do this API query on test.wikipedia.org.
A sample result is attached to this bug.
You can also import that file into your own wiki to do testing.
The Exif module that handles bitmaps seems to have the fields tagged correctly as Exif::RATIONAL, so I'm not sure exactly what is going wrong; whether it's when the file is uploaded or how we render results in the API.