Some file types aren’t allowed, like .xlsx. Where can I find a list of all of the file types that are allowed?
Per the uploader dialog (as of March 21, 2013), the allowed file types (which in this case really means ‘file extension’…) are:
- Image formats:
- JPEG (.jpg, .jpeg)
- GIF (.gif)
- BMP (.bmp)
- PNG (.png)
- TIFF (.tiff) (Really!? There’s no good reason to upload a tiff to this site!)
- ODF formats:
-
Most common
- Open Document Text (.odt)
- Open Document Spreadsheet (.ods)
- Open Document Presentation (.odp)
- Open Document Base (.odb)
-
Less common ODF formats
- Open Document Graphics (.odg)
- Open Document Chart (.odc)
- Open Document Formula (.odf)
- Open Document Image (.odi)
- Open Document Master (.odm)
- Open Document Text Template (.ott)
- Open Document Spreadsheet Template (.ots)
- Open Document Presentation Template (.otp)
- Open Document Graphics/Drawing Template (.otg)
-
Most common
But now “allowed file types are ‘.jpg’, ‘.jpeg’, ‘.gif’, ‘.bmp’, ‘.png’, ‘.tiff’” 
Update (2014-02-20)
fdo#52553 · Status: RESOLVED FIXED
Currently allowed file types are:
.jpg · .jpeg · .gif · .bmp · .png · .tiff
.odt · .ods · .odp · .odg · .odc · .odf · .odi · .odm · .ott · .ots · .otp · .otg · .odb
.doc · .docx · .xls · .xlsx · .ppt · .pptx
Maximum upload file size is 1048576K.