Just started using version .18. I saved a file but it didn't append a suffix. Looked at the file types at the bottom of the page and only choice was FreeCad Document. No pull down at all. And when I looked for the files using "Open" none of the files in the directory were listed. I did find a similar problem mention here and the solution was typing the suffix with the filename. That did allow me to save and also to see the files. Strangely, it also fixed the file type problem with the pulldown.
So I'm good. If I should do a bug report, let me know.
OS: Ubuntu 18.04.1 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.15187 (Git) AppImage
Build type: Release
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)