Fix file download from drag-and-drop on nuxeo-document-preview

Issue seems to be a regression from ELEMENTS-1534 as of testing with nuxeo-web-ui-3.0.19 and nuxeo-2021-HF33.

Uploading a file using the dropzone of nuxeo-document-preview results in the file being downloaded if of type .json, .html, .eml, or .msg.

Steps to Reproduce:

1. Create a File document with no attachment / main blob
2. From the document View tab, drag-and-drop a file of one of the affected types (.json, .html, .eml, or .msg) to the dropzone

Expected result: file is uploaded without an extraneous download being performed.

Actual result: after uploading file, a copy of the file is downloaded as well.