Just saved some files to Saved Messages in DC and found that it seems to truncate long file names, possibly at 34 characters in length?
as far as i know, there is no length truncation, however, filenames may get messed up due to other reasons under some circumstances.
lots of things have been fixed in this area (see “fix decoding of attachment filenames” in CHANGELOG), however, the fixes have not been released in an official release yet (but they are available in the nightlies for testing).
so, this should be tried over with a recent release, with chance, the issue is just gone 
I have the latest version from the Google Play Store installed. In any case, it is easy enough for the Android developers to confirm this bug (or not) and to verify, if correct, it has been fixed in a later nightly.