This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
Acorn originally reallocated the File-type ranges for applications. The new ranges are :-
Non-user area &E00-&FFF Acorn/RISCOS Ltd &B00-&DFF Commercial software &A00-&AFF Acornsoft, and other commercial software &400-&9FF Commercial Software
User area &100-&3FF Non-commercial distributed software (ie PD) &000-&0FF User's personal usage (ie non-distributed)
75% of the user area is for PD/Shareware, with allocations co-ordinated by Pineapple. If your software is going to be distributed, you should have an allocated filetype to avoid clashes.
RISCOS Ltd cannot publish its master list of filetypes because, at any given time, it will contain allocations made for products which have not yet been announced. Therefore, they would be in breach of confidence by doing so - and the editing overhead for producing a sanitised list is too great.
However, all is not lost. Denis Howe maintains an unofficial list of filetype allocations. This can be found at http://wombat.doc.ic.ac.uk/acorn/doc/filetypes and includes details about whether the allocation is an official one, a de facto one and what the file contains. If you would like an official 'Acorn' filetype issued to you, you should contact Pineapple Software, who are responsible for allocating filetypes.