Add check to preprocess file link hook to not run on webforms

This is a bugfix for the hook introduced in !159 (merged). This hook runs when files are uploaded to webforms which was something I didn't consider. My guess is that since files uploaded in webforms are stored privately, the hook isn't able to grab the string like it would with a publicly stored file. This creates an issue with appending (opens in a new window) to the filename. Since this feature isn't necessary in webforms, I've added a check to skip the hook if the user is currently on a webform.

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assigned to @canini.16

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merged

mentioned in commit 72763e13

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