Fix save file name with '.'s in it (#1773)

Using QFileInfo::baseName() discards everything after the first ., so
filenames like 1.2.3.4.png would be changed to 1.png. (See #1722).

Changing this to QFileInfo::completeBaseName() only discards the suffix,
so we have 1.2.3.4.jpg becoming 1.2.3.4.png, for example.