The datatype "fastqsanger.gz" can also be assigned. This keeps the data in compressed format, reducing history/dataset content size that counts in account quota size. Be sure to not simply assign "fastqsanger" - tools will not then know that the data is uncompressed.
This is brand-new functionality related with the upcoming 17.01 update Galaxy release. Documentation about usage will be updated once all behavior tunings are complete, around the time of the official release (estimate: few weeks).
Fastqsanger format is required for tools that interpret quality scores for calculations. Confirming fastqsanger (Sanger Phred +33) format is important or low mapping rates will results (and sometimes errors, depending on the tool). How to check format/current quality score scaling in uploaded datasets with FastQC, and how to rescale (if needed) with the Fastq Groomer tool, is explained here: https://wiki.galaxyproject.org/Support#FASTQ_Datatype_QA