Extension allowed for the Document Viewer

AFAIK, this is PDF and image formats. In case you hope for Office formats, you will be disappointed.

There is no list of supported extensions provided by Appian as such, but PDF files are always supported for preview. Can you tell us about your use case, I'm sure there can be some workarounds.

Hi [mention:37c57cf95dd64be9bdd8aad091affc6a:e9ed411860ed4f2ba0265705b8793d05]

As mentioned, you can display only PDF and image formats. However, if you want to display/preview any other formats you need to convert that doc into pdf for that instance and display. We do have few plugins to do that like : Plug-In

Despite what others here have already said: you can also open Text files (i.e. ".txt", aka plaintext) in the Document Viewer field.  This is the only common file format (other than images) that I've confirmed can be previewed, beyond PDF files.

FWIW, For image files, I always recommend using a!imageField() instead (they'll open in the Document Preview field but it works a little weirdly).  Some time ago I developed a tool that you can pass a file into, and it automatically checks whether the file type is previewable (and whether it's an image), and chooses the appropriate viewer.  If it's not a previewable type, it simply displays a download link.